java.lang.ClassCastException: java.lang.String cannot be cast to org.springframework.security.core.userdetails.User

This error occurs when you try to cast a String object to a User object in Spring Security. This is likely happening because you are trying to retrieve a User object from some source (such as a database or cache) and are getting back a String instead.

To fix this error, you should make sure that you are retrieving the correct object from the source. If you are using a database, make sure that you are selecting the User object and not just a String representation of it. If you are using a cache, make sure that you are storing and retrieving the User object correctly.

Another possible cause of this error is that you are trying to cast a different object to a User object. Make sure that you are casting the correct object to the User class.