[SOLVED] Deploying Keycloak template breaks the server (Migration to Quarkus)

I'm trying to migrate a custom template from pre-Quarkus to Keycloak Quarkus (24.0.1). It is located at <KEYCLOAK_HOME>/themes/base/login/consent-required-action.ftl.

The template looks as follows:

<#import "template.ftl" as layout>
<@layout.registrationLayout; section>
    <#if section = "title">
        ${msg("loginTitle",realm.name)}
    <#elseif section = "header">
        ${msg("loginTitleHtml",realm.name)}
    <#elseif section = "form">
        <form id="kc-totp-login-form" class="${properties.kcFormClass!}" action="${url.loginAction}" method="post">
            <div class="${properties.kcFormGroupClass!}">
                <div class="${properties.kcLabelWrapperClass!}">
                    <label for="totp" class="${properties.kcLabelClass!}">
                        In order to proceed we need permission to load information about your username to our servers. Otherwise you won't be able to use our IT services.
                    </label>
                </div>
            </div>

            <div class="${properties.kcFormGroupClass!}">
                <div id="kc-form-options" class="${properties.kcFormOptionsClass!}">
                    <div class="${properties.kcFormOptionsWrapperClass!}">
                    </div>
                </div>

                <div id="kc-form-buttons" class="${properties.kcFormButtonsClass!}">
                    <div class="${properties.kcFormButtonsWrapperClass!}">
                        <input class="${properties.kcButtonClass!} ${properties.kcButtonPrimaryClass!} ${properties.kcButtonLargeClass!}" name="accept" id="kc-accept" type="submit" value="${msg("doAccept")}"/>
                        <input class="${properties.kcButtonClass!} ${properties.kcButtonDefaultClass!} ${properties.kcButtonLargeClass!}" name="decline" id="kc-decline" type="submit" value="${msg("doDecline")}"/>
                    </div>
                </div>
            </div>
        </form>
    </#if>
</@layout.registrationLayout>

As soon as this template is uploaded and the web-interface is accessed it gives an "Internal Server Error" on the website and the keycloak-service logs the following error:

2024-03-27 18:46:36,383 ERROR [org.keycloak.forms.login.freemarker.FreeMarkerLoginFormsProvider] (executor-thread-2) Failed to process template: org.keycloak.theme.FreeMarkerException: Failed to process template login.ftl
        at org.keycloak.theme.freemarker.DefaultFreeMarkerProvider.processTemplate(DefaultFreeMarkerProvider.java:52)
        at org.keycloak.forms.login.freemarker.FreeMarkerLoginFormsProvider.processTemplate(FreeMarkerLoginFormsProvider.java:544)
        at org.keycloak.forms.login.freemarker.FreeMarkerLoginFormsProvider.createResponse(FreeMarkerLoginFormsProvider.java:316)
        at org.keycloak.forms.login.freemarker.FreeMarkerLoginFormsProvider.createLoginUsernamePassword(FreeMarkerLoginFormsProvider.java:558)
        at org.keycloak.authentication.authenticators.browser.UsernamePasswordForm.challenge(UsernamePasswordForm.java:95)
        at org.keycloak.authentication.authenticators.browser.UsernamePasswordForm.authenticate(UsernamePasswordForm.java:81)
        at org.keycloak.authentication.DefaultAuthenticationFlow.processSingleFlowExecutionModel(DefaultAuthenticationFlow.java:442)
        at org.keycloak.authentication.DefaultAuthenticationFlow.processFlow(DefaultAuthenticationFlow.java:246)
        at org.keycloak.authentication.DefaultAuthenticationFlow.processSingleFlowExecutionModel(DefaultAuthenticationFlow.java:377)
        at org.keycloak.authentication.DefaultAuthenticationFlow.processFlow(DefaultAuthenticationFlow.java:268)
        at org.keycloak.authentication.AuthenticationProcessor.authenticateOnly(AuthenticationProcessor.java:1027)
        at org.keycloak.protocol.AuthorizationEndpointBase.handleBrowserAuthenticationRequest(AuthorizationEndpointBase.java:116)
        at org.keycloak.protocol.oidc.endpoints.AuthorizationEndpoint.buildAuthorizationCodeAuthorizationResponse(AuthorizationEndpoint.java:337)
        at org.keycloak.protocol.oidc.endpoints.AuthorizationEndpoint.process(AuthorizationEndpoint.java:202)
        at org.keycloak.protocol.oidc.endpoints.AuthorizationEndpoint.buildGet(AuthorizationEndpoint.java:113)
        at org.keycloak.protocol.oidc.endpoints.AuthorizationEndpoint$quarkusrestinvoker$buildGet_4b690b27439f19dd29733dc5fd4004f24de0adb6.invoke(Unknown Source)
        at org.jboss.resteasy.reactive.server.handlers.InvocationHandler.handle(InvocationHandler.java:29)
        at io.quarkus.resteasy.reactive.server.runtime.QuarkusResteasyReactiveRequestContext.invokeHandler(QuarkusResteasyReactiveRequestContext.java:141)
        at org.jboss.resteasy.reactive.common.core.AbstractResteasyReactiveContext.run(AbstractResteasyReactiveContext.java:147)
        at io.quarkus.vertx.core.runtime.VertxCoreRecorder$14.runWith(VertxCoreRecorder.java:582)
        at org.jboss.threads.EnhancedQueueExecutor$Task.run(EnhancedQueueExecutor.java:2513)
        at org.jboss.threads.EnhancedQueueExecutor$ThreadBody.run(EnhancedQueueExecutor.java:1538)
        at org.jboss.threads.DelegatingRunnable.run(DelegatingRunnable.java:29)
        at org.jboss.threads.ThreadLocalResettingRunnable.run(ThreadLocalResettingRunnable.java:29)
        at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
        at java.base/java.lang.Thread.run(Thread.java:840)
Caused by: freemarker.template.TemplateNotFoundException: Template not found for name "login.ftl".
The name was interpreted by this TemplateLoader: org.keycloak.theme.freemarker.DefaultFreeMarkerProvider$ThemeTemplateLoader@4579404f.
        at freemarker.template.Configuration.getTemplate(Configuration.java:2957)
        at freemarker.template.Configuration.getTemplate(Configuration.java:2777)
        at org.keycloak.theme.freemarker.DefaultFreeMarkerProvider.getTemplate(DefaultFreeMarkerProvider.java:66)
        at org.keycloak.theme.freemarker.DefaultFreeMarkerProvider.processTemplate(DefaultFreeMarkerProvider.java:45)
        ... 25 more
2024-03-27 18:46:36,922 ERROR [org.keycloak.forms.login.freemarker.FreeMarkerLoginFormsProvider] (executor-thread-2) Failed to process template: org.keycloak.theme.FreeMarkerException: Failed to process template login.ftl
        at org.keycloak.theme.freemarker.DefaultFreeMarkerProvider.processTemplate(DefaultFreeMarkerProvider.java:52)
        at org.keycloak.forms.login.freemarker.FreeMarkerLoginFormsProvider.processTemplate(FreeMarkerLoginFormsProvider.java:544)
        at org.keycloak.forms.login.freemarker.FreeMarkerLoginFormsProvider.createResponse(FreeMarkerLoginFormsProvider.java:316)
        at org.keycloak.forms.login.freemarker.FreeMarkerLoginFormsProvider.createLoginUsernamePassword(FreeMarkerLoginFormsProvider.java:558)
        at org.keycloak.authentication.authenticators.browser.UsernamePasswordForm.challenge(UsernamePasswordForm.java:95)
        at org.keycloak.authentication.authenticators.browser.UsernamePasswordForm.authenticate(UsernamePasswordForm.java:81)
        at org.keycloak.authentication.DefaultAuthenticationFlow.processSingleFlowExecutionModel(DefaultAuthenticationFlow.java:442)
        at org.keycloak.authentication.DefaultAuthenticationFlow.processFlow(DefaultAuthenticationFlow.java:246)
        at org.keycloak.authentication.DefaultAuthenticationFlow.processSingleFlowExecutionModel(DefaultAuthenticationFlow.java:377)
        at org.keycloak.authentication.DefaultAuthenticationFlow.processFlow(DefaultAuthenticationFlow.java:268)
        at org.keycloak.authentication.AuthenticationProcessor.authenticateOnly(AuthenticationProcessor.java:1027)
        at org.keycloak.authentication.AuthenticationProcessor.authenticate(AuthenticationProcessor.java:884)
        at org.keycloak.protocol.AuthorizationEndpointBase.handleBrowserAuthenticationRequest(AuthorizationEndpointBase.java:152)
        at org.keycloak.protocol.oidc.endpoints.AuthorizationEndpoint.buildAuthorizationCodeAuthorizationResponse(AuthorizationEndpoint.java:337)
        at org.keycloak.protocol.oidc.endpoints.AuthorizationEndpoint.process(AuthorizationEndpoint.java:202)
        at org.keycloak.protocol.oidc.endpoints.AuthorizationEndpoint.buildGet(AuthorizationEndpoint.java:113)
        at org.keycloak.protocol.oidc.endpoints.AuthorizationEndpoint$quarkusrestinvoker$buildGet_4b690b27439f19dd29733dc5fd4004f24de0adb6.invoke(Unknown Source)
        at org.jboss.resteasy.reactive.server.handlers.InvocationHandler.handle(InvocationHandler.java:29)
        at io.quarkus.resteasy.reactive.server.runtime.QuarkusResteasyReactiveRequestContext.invokeHandler(QuarkusResteasyReactiveRequestContext.java:141)
        at org.jboss.resteasy.reactive.common.core.AbstractResteasyReactiveContext.run(AbstractResteasyReactiveContext.java:147)
        at io.quarkus.vertx.core.runtime.VertxCoreRecorder$14.runWith(VertxCoreRecorder.java:582)
        at org.jboss.threads.EnhancedQueueExecutor$Task.run(EnhancedQueueExecutor.java:2513)
        at org.jboss.threads.EnhancedQueueExecutor$ThreadBody.run(EnhancedQueueExecutor.java:1538)
        at org.jboss.threads.DelegatingRunnable.run(DelegatingRunnable.java:29)
        at org.jboss.threads.ThreadLocalResettingRunnable.run(ThreadLocalResettingRunnable.java:29)
        at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
        at java.base/java.lang.Thread.run(Thread.java:840)
Caused by: freemarker.template.TemplateNotFoundException: Template not found for name "login.ftl".
The name was interpreted by this TemplateLoader: org.keycloak.theme.freemarker.DefaultFreeMarkerProvider$ThemeTemplateLoader@29e1cb6d.
        at freemarker.template.Configuration.getTemplate(Configuration.java:2957)
        at freemarker.template.Configuration.getTemplate(Configuration.java:2777)
        at org.keycloak.theme.freemarker.DefaultFreeMarkerProvider.getTemplate(DefaultFreeMarkerProvider.java:66)
        at org.keycloak.theme.freemarker.DefaultFreeMarkerProvider.processTemplate(DefaultFreeMarkerProvider.java:45)
        ... 26 more
2024-03-27 18:46:36,923 ERROR [org.keycloak.headers.DefaultSecurityHeadersProvider] (executor-thread-2) MediaType not set on path /auth/realms/master/protocol/openid-connect/auth, with response status 500
2024-03-27 18:46:36,924 ERROR [org.keycloak.services.error.KeycloakErrorHandler] (executor-thread-2) Uncaught server error: jakarta.ws.rs.InternalServerErrorException: HTTP 500 Internal Server Error
        at org.keycloak.headers.DefaultSecurityHeadersProvider.addHeaders(DefaultSecurityHeadersProvider.java:75)
        at org.keycloak.services.filters.KeycloakSecurityHeadersFilter.filter(KeycloakSecurityHeadersFilter.java:43)
        at org.jboss.resteasy.reactive.server.handlers.ResourceResponseFilterHandler.handle(ResourceResponseFilterHandler.java:25)
        at io.quarkus.resteasy.reactive.server.runtime.QuarkusResteasyReactiveRequestContext.invokeHandler(QuarkusResteasyReactiveRequestContext.java:150)
        at org.jboss.resteasy.reactive.common.core.AbstractResteasyReactiveContext.run(AbstractResteasyReactiveContext.java:147)
        at io.quarkus.vertx.core.runtime.VertxCoreRecorder$14.runWith(VertxCoreRecorder.java:582)
        at org.jboss.threads.EnhancedQueueExecutor$Task.run(EnhancedQueueExecutor.java:2513)
        at org.jboss.threads.EnhancedQueueExecutor$ThreadBody.run(EnhancedQueueExecutor.java:1538)
        at org.jboss.threads.DelegatingRunnable.run(DelegatingRunnable.java:29)
        at org.jboss.threads.ThreadLocalResettingRunnable.run(ThreadLocalResettingRunnable.java:29)
        at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
        at java.base/java.lang.Thread.run(Thread.java:840)
2024-03-27 18:46:36,927 ERROR [org.keycloak.services.error.KeycloakErrorHandler] (executor-thread-2) Failed to create error page: org.keycloak.theme.FreeMarkerException: Failed to process template error.ftl
        at org.keycloak.theme.freemarker.DefaultFreeMarkerProvider.processTemplate(DefaultFreeMarkerProvider.java:52)
        at org.keycloak.services.error.KeycloakErrorHandler.getResponse(KeycloakErrorHandler.java:108)
        at org.keycloak.services.error.KeycloakErrorHandler.toResponse(KeycloakErrorHandler.java:67)
        at org.jboss.resteasy.reactive.server.core.RuntimeExceptionMapper.mapException(RuntimeExceptionMapper.java:100)
        at org.jboss.resteasy.reactive.server.core.ResteasyReactiveRequestContext.mapExceptionIfPresent(ResteasyReactiveRequestContext.java:346)
        at org.jboss.resteasy.reactive.server.handlers.ExceptionHandler.handle(ExceptionHandler.java:15)
        at io.quarkus.resteasy.reactive.server.runtime.QuarkusResteasyReactiveRequestContext.invokeHandler(QuarkusResteasyReactiveRequestContext.java:150)
        at org.jboss.resteasy.reactive.common.core.AbstractResteasyReactiveContext.run(AbstractResteasyReactiveContext.java:147)
        at io.quarkus.vertx.core.runtime.VertxCoreRecorder$14.runWith(VertxCoreRecorder.java:582)
        at org.jboss.threads.EnhancedQueueExecutor$Task.run(EnhancedQueueExecutor.java:2513)
        at org.jboss.threads.EnhancedQueueExecutor$ThreadBody.run(EnhancedQueueExecutor.java:1538)
        at org.jboss.threads.DelegatingRunnable.run(DelegatingRunnable.java:29)
        at org.jboss.threads.ThreadLocalResettingRunnable.run(ThreadLocalResettingRunnable.java:29)
        at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
        at java.base/java.lang.Thread.run(Thread.java:840)
Caused by: freemarker.template.TemplateNotFoundException: Template not found for name "error.ftl".
The name was interpreted by this TemplateLoader: org.keycloak.theme.freemarker.DefaultFreeMarkerProvider$ThemeTemplateLoader@74524fda.
        at freemarker.template.Configuration.getTemplate(Configuration.java:2957)
        at freemarker.template.Configuration.getTemplate(Configuration.java:2777)
        at org.keycloak.theme.freemarker.DefaultFreeMarkerProvider.getTemplate(DefaultFreeMarkerProvider.java:66)
        at org.keycloak.theme.freemarker.DefaultFreeMarkerProvider.processTemplate(DefaultFreeMarkerProvider.java:45)
        ... 14 more
2024-03-27 18:46:36,927 ERROR [org.keycloak.headers.DefaultSecurityHeadersProvider] (executor-thread-2) MediaType not set on path /auth/realms/master/protocol/openid-connect/auth, with response status 500
2024-03-27 18:46:36,930 ERROR [io.quarkus.vertx.http.runtime.QuarkusErrorHandler] (executor-thread-2) HTTP Request to /auth/realms/master/protocol/openid-connect/auth?client_id=<redacted>&redirect_uri=<redacted>&response_mode=fragment&response_type=code&scope=openid&nonce=<redacted>1&code_challenge=<redacted>&code_challenge_method=S256 failed, error id: 92f86fe9-2438-4a5b-9741-3a95fb53103b-1: jakarta.ws.rs.InternalServerErrorException: HTTP 500 Internal Server Error
        at org.keycloak.headers.DefaultSecurityHeadersProvider.addHeaders(DefaultSecurityHeadersProvider.java:75)
        at org.keycloak.services.filters.KeycloakSecurityHeadersFilter.filter(KeycloakSecurityHeadersFilter.java:43)
        at org.jboss.resteasy.reactive.server.handlers.ResourceResponseFilterHandler.handle(ResourceResponseFilterHandler.java:25)
        at io.quarkus.resteasy.reactive.server.runtime.QuarkusResteasyReactiveRequestContext.invokeHandler(QuarkusResteasyReactiveRequestContext.java:150)
        at org.jboss.resteasy.reactive.common.core.AbstractResteasyReactiveContext.run(AbstractResteasyReactiveContext.java:147)
        at io.quarkus.vertx.core.runtime.VertxCoreRecorder$14.runWith(VertxCoreRecorder.java:582)
        at org.jboss.threads.EnhancedQueueExecutor$Task.run(EnhancedQueueExecutor.java:2513)
        at org.jboss.threads.EnhancedQueueExecutor$ThreadBody.run(EnhancedQueueExecutor.java:1538)
        at org.jboss.threads.DelegatingRunnable.run(DelegatingRunnable.java:29)
        at org.jboss.threads.ThreadLocalResettingRunnable.run(ThreadLocalResettingRunnable.java:29)
        at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
        at java.base/java.lang.Thread.run(Thread.java:840)

What is strange is that this happens without any provider uploaded that uses this template, nor the template being selected in the web-interface. Also the error fails to process, not the template consent-required-action.ftl or template.ftl which is imported in the first, but rather it fails to process login.ftl which is seemingly unrelated (Not imported in consent-required-action.ftl nor in template.ftl).

I checked if the referenced theme template.ftl is also available in the Quarkus version, and while it has been moved from <KEYCLOAK_HOME>/themes/base/login/template.ftl to the jar located at <KEYCLOAK_HOME>/lib/lib/main/org.keycloak.keycloak-themes-24.0.1.jar in which it has the path theme/base/login/template.ftl, it does still exist and has no notable changes.