Fat jar from scala project throws error when run

I created Scala project using Maven as build tools. I try to build fat jar that later I will be able to run from command line. I wrote a program that use Keycloak library to create users with REST API. It works fine when I run it straight from the Intellij but when I compile and run jar it gives me following error which I assume is result of me misconfiguring maven and not including all required dependencies?

Exception in thread "main" javax.ws.rs.ProcessingException: RESTEASY004655: Unable to invoke request: javax.ws.rs.ProcessingException: RESTEASY003215: could not find writer for content-type application/x-www-form-urlencoded type: javax.ws.rs.core.Form$1
    at org.jboss.resteasy.client.jaxrs.engines.ApacheHttpClient4Engine.invoke(ApacheHttpClient4Engine.java:328)
[...]

My pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>org.generator</groupId>
  <artifactId>data-generator</artifactId>
  <version>1.0-SNAPSHOT</version>

  <properties>
    <exec.mainClass>com.generator.GeneratorApp</exec.mainClass>
    <maven.compiler.source>1.8</maven.compiler.source>
    <maven.compiler.target>1.8</maven.compiler.target>
    <encoding>UTF-8</encoding>
    <scala.version>2.13.8</scala.version>
  </properties>

  <dependencies>
    <dependency>
      <groupId>org.scala-lang</groupId>
      <artifactId>scala-library</artifactId>
      <version>${scala.version}</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.keycloak</groupId>
      <artifactId>keycloak-admin-client</artifactId>
      <version>18.0.2</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.postgresql</groupId>
      <artifactId>postgresql</artifactId>
      <version>42.4.0</version>
      <scope>compile</scope>
    </dependency>
  </dependencies>

  <build>
    <sourceDirectory>src/main/scala</sourceDirectory>
    <plugins>
      <plugin>
        <groupId>net.alchim31.maven</groupId>
        <artifactId>scala-maven-plugin</artifactId>
        <version>3.3.2</version>
        <executions>
          <execution>
            <goals>
              <goal>compile</goal>
              <goal>testCompile</goal>
            </goals>
            <configuration>
              <args>
                <arg>-dependencyfile</arg>
                <arg>${project.build.directory}/.scala_dependencies</arg>
              </args>
            </configuration>
          </execution>
        </executions>
      </plugin>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-surefire-plugin</artifactId>
        <version>2.21.0</version>
        <configuration>
          <skipTests>true</skipTests>
        </configuration>
      </plugin>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-assembly-plugin</artifactId>
        <version>2.4</version>
        <configuration>
          <descriptorRefs>
            <descriptorRef>jar-with-dependencies</descriptorRef>
          </descriptorRefs>
          <archive>
            <manifest>
              <addClasspath>true</addClasspath>
              <mainClass>com.generator.GeneratorApp</mainClass>
            </manifest>
          </archive>
        </configuration>
        <executions>
          <execution>
            <phase>package</phase>
            <goals>
              <goal>single</goal>
            </goals>
          </execution>
        </executions>
      </plugin>
    </plugins>
  </build>
</project>

And commands I am using to compile and run:

mvn clean compile assembly:single     

java -cp ./target/data-generator-1.0-SNAPSHOT-jar-with-dependencies.jar com.generator.GeneratorApp

My experience with Maven is very limited (usually i was using SBT when working with Scala). I googled and searched through StackOverflow but I am stuck on this for some time now.

Let me know if you need any more details. I tried to include everything I thought is relevant.

In my case, to fix it, I had to include following dependencies in pom.xml:

    <dependency>
      <groupId>org.jboss.resteasy</groupId>
      <artifactId>resteasy-jaxrs</artifactId>
      <version>3.13.2.Final</version>
    </dependency>

    <dependency>
      <groupId>org.jboss.resteasy</groupId>
      <artifactId>resteasy-jaxb-provider</artifactId>
      <version>3.13.2.Final</version>
    </dependency>

    <dependency>
      <groupId>org.jboss.resteasy</groupId>
      <artifactId>resteasy-jackson2-provider</artifactId>
      <version>3.13.2.Final</version>
    </dependency>

Not sure why I need it, I thought they would be included automatically based on my other dependencies (Keycloak), which makes me think that it is workaround not actual solution.