Error loading dynamic type: [Web.TPEngine.Providers.RestfulProvider]; Diagnostics=

I'm implementing a custom policy (ContactSignUp.xml) in Azure AD B2C that calls a REST API (ValidateInvite in InviteController) when a user clicks an email link to sign up. The inviteToken is passed as input. The API is configured via the REST-GetEmailFromInviteToken technical profile.

However, during execution, I get the error:

Error loading dynamic type: [Web.TPEngine.Providers.RestfulProvider]; Diagnostics=. The REST API appears unreachable from the B2C policy.

<BuildingBlocks>
  <ClaimsSchema>
    <ClaimType Id="email">
      <DisplayName>Email Address</DisplayName>
      <DataType>string</DataType>
      <DefaultPartnerClaimTypes>
        <Protocol Name="OAuth2" PartnerClaimType="email"/>
      </DefaultPartnerClaimTypes>
    </ClaimType>
    <ClaimType Id="inviteToken">
      <DisplayName>Invite Token</DisplayName>
      <DataType>string</DataType>
      <DefaultPartnerClaimTypes>
        <Protocol Name="OAuth2" PartnerClaimType="extension_inviteToken"/>
      </DefaultPartnerClaimTypes>
    </ClaimType>
    <ClaimType Id="accountName">
      <DisplayName>Account Name</DisplayName>
      <DataType>string</DataType>
    </ClaimType>
    <ClaimType Id="contactName">
      <DisplayName>Contact Name</DisplayName>
      <DataType>string</DataType>
    </ClaimType>
    <ClaimType Id="newPassword">
      <DisplayName>New Password</DisplayName>
      <DataType>string</DataType>
    </ClaimType>
    <ClaimType Id="reenterPassword">
      <DisplayName>Reenter Password</DisplayName>
      <DataType>string</DataType>
    </ClaimType>
    <ClaimType Id="objectId">
      <DisplayName>Object Id</DisplayName>
      <DataType>string</DataType>
    </ClaimType>
    <ClaimType Id="tenantId">
      <DisplayName>Tenant Id</DisplayName>
      <DataType>string</DataType>
    </ClaimType>
  </ClaimsSchema>
</BuildingBlocks>
<ClaimsProviders>
  <ClaimsProvider>
    <DisplayName>REST API</DisplayName>
    <TechnicalProfiles>
      <TechnicalProfile Id="REST-GetEmailFromInviteToken">
        <DisplayName>Get Email From Invite Token</DisplayName>
        <Protocol Name="Proprietary" Handler="Web.TPEngine.Providers.RestfulProvider, Web.TPEngine, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null"/>
        <Metadata>
          <Item Key="ServiceUrl">https://api7267.ngrok.io/api/invite/ValidateInvite</Item>
          <Item Key="SendClaimsIn">Body</Item>
          <Item Key="AuthenticationType">None</Item>
        </Metadata>
        <InputClaims>
          <InputClaim ClaimTypeReferenceId="inviteToken" Required="true"/>
        </InputClaims>
        <OutputClaims>
          <OutputClaim ClaimTypeReferenceId="email"/>
          <OutputClaim ClaimTypeReferenceId="accountName"/>
          <OutputClaim ClaimTypeReferenceId="contactName"/>
        </OutputClaims>
      </TechnicalProfile>
    </TechnicalProfiles>
  </ClaimsProvider>
  <ClaimsProvider>
    <DisplayName>Self Asserted</DisplayName>
    <TechnicalProfiles>
      <TechnicalProfile Id="SelfAsserted-LocalAccountSignUp-ReadOnlyEmail">
        <DisplayName>Local Account Sign Up (ReadOnly Email)</DisplayName>
        <Protocol Name="Proprietary" Handler="Web.TPEngine.Providers.SelfAssertedAttributeProvider"/>
        <Metadata>
          <Item Key="ContentDefinitionReferenceId">api.localaccountsignup</Item>
          <Item Key="ReadOnlyClaimNames">email,accountName,contactName</Item>
        </Metadata>
        <InputClaims>
          <InputClaim ClaimTypeReferenceId="email"/>
          <InputClaim ClaimTypeReferenceId="inviteToken"/>
          <InputClaim ClaimTypeReferenceId="accountName"/>
          <InputClaim ClaimTypeReferenceId="contactName"/>
        </InputClaims>
        <OutputClaims>
          <OutputClaim ClaimTypeReferenceId="email" Required="true"/>
          <OutputClaim ClaimTypeReferenceId="newPassword" Required="true"/>
          <OutputClaim ClaimTypeReferenceId="reenterPassword" Required="true"/>
          <OutputClaim ClaimTypeReferenceId="inviteToken"/>
          <OutputClaim ClaimTypeReferenceId="accountName"/>
          <OutputClaim ClaimTypeReferenceId="contactName"/>
        </OutputClaims>
      </TechnicalProfile>
    </TechnicalProfiles>
  </ClaimsProvider>
  <ClaimsProvider>
    <DisplayName>Azure Active Directory</DisplayName>
    <TechnicalProfiles>
      <TechnicalProfile Id="AAD-UserWriteUsingLogonEmail">
        <Metadata>
          <Item Key="Operation">Write</Item>
          <Item Key="RaiseErrorIfClaimsPrincipalAlreadyExists">true</Item>
        </Metadata>
        <InputClaims>
          <InputClaim ClaimTypeReferenceId="email" PartnerClaimType="signInName" Required="true"/>
          <InputClaim ClaimTypeReferenceId="newPassword" Required="true"/>
        </InputClaims>
        <PersistedClaims>
          <PersistedClaim ClaimTypeReferenceId="email" PartnerClaimType="signInName"/>
          <PersistedClaim ClaimTypeReferenceId="newPassword"/>
        </PersistedClaims>
        <OutputClaims>
          <OutputClaim ClaimTypeReferenceId="objectId"/>
          <OutputClaim ClaimTypeReferenceId="email" PartnerClaimType="signInName"/>
        </OutputClaims>
        <IncludeTechnicalProfile ReferenceId="AAD-Common"/>
      </TechnicalProfile>
      <TechnicalProfile Id="AAD-Common">
        <Protocol Name="Proprietary" Handler="Web.TPEngine.Providers.AadBackendProvider"/>
      </TechnicalProfile>
    </TechnicalProfiles>
  </ClaimsProvider>
</ClaimsProviders>
<UserJourneys>
  <UserJourney Id="SignUpWithInviteToken">
    <OrchestrationSteps>
      <OrchestrationStep Order="1" Type="ClaimsExchange">
        <ClaimsExchanges>
          <ClaimsExchange Id="GetEmailFromInviteToken" TechnicalProfileReferenceId="REST-GetEmailFromInviteToken"/>
        </ClaimsExchanges>
      </OrchestrationStep>
      <OrchestrationStep Order="2" Type="ClaimsExchange">
        <ClaimsExchanges>
          <ClaimsExchange Id="SelfAssertedSignUp" TechnicalProfileReferenceId="SelfAsserted-LocalAccountSignUp-ReadOnlyEmail"/>
        </ClaimsExchanges>
      </OrchestrationStep>
      <OrchestrationStep Order="3" Type="ClaimsExchange">
        <ClaimsExchanges>
          <ClaimsExchange Id="CreateAADAccount" TechnicalProfileReferenceId="AAD-UserWriteUsingLogonEmail"/>
        </ClaimsExchanges>
      </OrchestrationStep>
      <OrchestrationStep Order="4" Type="SendClaims" CpimIssuerTechnicalProfileReferenceId="JwtIssuer"/>
    </OrchestrationSteps>
  </UserJourney>
</UserJourneys>

This could be caused by a few things. I would start with checking the following:

• Network connectivity - Azure B2C requires your ServiceUrl to be publicly accessible. Test this using curl, like so:

  curl -X POST https://api7267.ngrok.io/api/invite/ValidateInvite -H "Content-Type: application/json" -d '{"inviteToken":"test123"}'

  If this is not working, make sure your NGrok tunnel is up and there are no IP-restrictions configured.

• SSL certificate - Azure B2C requires you use a valid and trusted SSL certificate on your API. A self-signed certificate will not work. Also check SSL certificate expiry date.