Custom Policy RESTful-Api UserJourney Error - azure

Question

I am developing a RESTful Service, which gets called during the Signup/Signin Process in Azure AD B2C. My Service logs state, that data successfully arrives, and output-claims (customerId) get created.
But I receive the following error message, and the user doesn't get created:
AADB2C90161 A self-asserted send response has failed with reason (Internal Server Error).
Correlation ID 7eac5fd2-cd85-4535-b166-4cc8f0264d07
I have oriented myself towards this example: https://github.com/Azure-Samples/active-directory-b2c-custom-policy-starterpack/tree/master/scenarios/aadb2c-ief-rest-api-netfw/
Did anyone experience similar issues and has a hint what could be the problem in my case?
in TrustFrameworkExtension:
<ClaimsProvider>
<DisplayName>KTM REST APIs</DisplayName>
<TechnicalProfiles>
<TechnicalProfile Id="REST-API-SignUp">
<DisplayName>Generate and return customerID claim</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://<my.service.com>/api/Identity/Signup</Item>
<Item Key="AuthenticationType">None</Item>
<Item Key="SendClaimsIn">Body</Item>
</Metadata>
<InputClaims>
<InputClaim ClaimTypeReferenceId="email" PartnerClaimType="Email" />
<InputClaim ClaimTypeReferenceId="givenName" PartnerClaimType="FirstName" />
<InputClaim ClaimTypeReferenceId="surname" PartnerClaimType="LastName" />
<InputClaim ClaimTypeReferenceId="testClaim" PartnerClaimType="ObjectId" />
</InputClaims>
<OutputClaims>
<OutputClaim ClaimTypeReferenceId="customerId" PartnerClaimType="CustomerId" />
</OutputClaims>
<UseTechnicalProfileForSessionManagement ReferenceId="SM-Noop" />
</TechnicalProfile>
<TechnicalProfile Id="LocalAccountSignUpWithLogonEmail">
<OutputClaims>
<OutputClaim ClaimTypeReferenceId="customerId" PartnerClaimType="CustomerId" />
</OutputClaims>
<ValidationTechnicalProfiles>
<ValidationTechnicalProfile ReferenceId="REST-API-SignUp" />
</ValidationTechnicalProfiles>
</TechnicalProfile>
</TechnicalProfiles>
</ClaimsProvider>

Answer 1

In this case, you have a run-time error. The claim type "customerId" is defined as a string in the policy, but looks like the value coming over the wire (where partnerClaimType is "CustomerId") is a number, so the system is unable to map it. See this line:
<OutputClaim ClaimTypeReferenceId="customerId" PartnerClaimType="CustomerId" />
This is how a Rest API will return a string vs a number (note the absence of quotes in a number):
{
"name": "John",
"age": 24
}
While this message can be improved, you should configure your policy to collect logs using Application Insights. This will allow you to debug similar run-time issues more easily.