OpenIddcit authorization flow

I hope you guys can help me. We have some Blazor web applications that used authorization based on Windows Authentication. Because we wanted to be independand of authentication method, we have an OpenIddict server that provides tokens to clients. This way we can easily port the clients to other means of authentication.

The flow seems to work allright and the server returns a valid access_token and id_token to the client.
The payload contains the following:

  "access_token": "<some-access-token>",
  "token_type": "Bearer",
  "expires_in": 3599,
  "scope": "openid",
  "id_token": "<some-id-token>"

OpenIddict Server:
The server signs in without problems, and returns a result with Identity.IsAuthenticated = true…

Blazor WASM Client:
The tokens are valid, signatures are verified and contain the right information for the client application. The client, however, never signs-in and redirects to login-failed with the message: “There was an error signing in”.

I have just started with OpenID / OAuth so I believe I am probably missing something completely obvious.

Any ideas, anyone?