AADSTS50146: This application is required to be configured with an application-specific signing key

I was testing the sign-in process for the OIDC flow using Postman. It worked fine on the day I set it up, but the next day I encountered an issue and I could not retrieve the ID token anymore. I was getting the following AADSTS50146 error.

Then I remembered that I had added an optional claim in the ID token for testing purposes.

The error description did not seem to match the actual root cause. I searched online and found a solution on this page: azure active directory – AADSTS50146 error when attempting to retrieve Oauth access_token – Stack Overflow

According to the answer, the problem was caused by the acceptMappedClaims property being set to null by default. When I followed the instructions to set it to true, the issue was resolved.

I also noticed that the optional claim is reflected in the ID token: