I have just created a new entra external ID tenant, I have setup custom authentication extensions for AttributeCollectionSubmit and TokenIssuanceStart. Those will call a REST api that lives in another tenant. I setup authentication and granted admin consent to the app registration in my main tenant. I also setup an identity provider in my external ID to federate to my main entra tenant so that I can test things.

I setup a user flow so that I can collect come fields during sign up. Once I teste the flow, I get to the sign up screen, when I press next I get ""Something went wrong. Contact your IT department if the problem persists.".

In edge/chrome, In developer tools I can see the error

"error": {

    "code": 6000,

    "correlationId": "f22236f8-4145-41a6-90ab-0118e4ed2f20",

    "timestamp": "2026-05-20 20:28:02Z",

    "isFatal": true,

    "message": "AADSTS1100001"

}

I have tested the API directly, as much as I know how to based on what I think the payloads are going to be. The API is callable and no errors and the logs show as expected.

I look in my APIs application insights, and no logs whatsoever despite me logging to app insights.

I have looked at sign-in logs in the external ID tenant and don't see any at all. It's swallowing the error and I have no idea how to troubleshoot at this point.

Looking for some help in troubleshooting and knowing what to look for.

Posted here as well: https://learn.microsoft.com/en-us/answers/questions/5897494/troubleshooting-microsoft-external-id-custom-authe