If you have implemented a feature or SDK application that retrieves the user id from the user context, you should ensure that you use:

 ApplicationServices.UserContext.UserId

and NOT the property

ApplicationServices.UserContext.UserName

The UserId and UserName property returned the same value in many system configurations until a few years ago when additional security configuration options were introduced. To be on the safe side and be future proof, the UserName property should be used for display purposes in the user interface and the UserId property used for REST calls and business functions.