"error_uri is null", An internal server error occurred

Error from Microsoft.AspNetCore.Authentication.OpenIdConnect.OpenIdConnectHandler: “developer.signicat. io/errors#internal-system-error An internal server error occurred” with message:
“Message contains error: ‘“sp:authsolution:internal-error”’, error_uri: ‘“error_uri is null”’.”

I cannot seem to locate where this error_uri is supposed to be set. Is this to be configured in my asp.net core application, or in the signicat dashboard?

As far as I can tell the OpenID Connect middleware in my asp.net core app is expecting an error uri to be returned in the error response, but this seems then not to be present.

Can anyone help?