.NET Application Suddenly Stopped Working : Certificate Expired

A situation where a .NET application can suddenly stop working with no changes.

The /callback endpoint will return a 404 with an error similar to the one below in their app server logs:

"2023-06-28 12:52:27,379","[7]","ERROR","An unhandled exception occurs.","Microsoft.IdentityModel.Protocols.OpenIdConnect.OpenIdConnectProtocolException: "OpenIdConnectMessage.Error was not null, indicating an error. Error: 'unauthorized'. Error_Description (may be empty): 'user is blocked'. Error_Uri (may be empty): 'error_uri is null'."

The security certificate may have expired.

 

1. Check if the site certificate has expired.
2. If expired, renew the certificate.