.NET Application Suddenly Stopped Working : Certificate Expired

Problem statement

A situation where using a .NET application, it can suddenly stop working with no changes introduced by the customer.

The /callback endpoint will return a 404 with an error similar to the 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 application suddenly stops working, for no obvious reason. The error message suggests that a user has been blocked ( or similar condition ).


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