My TwinCAT PLC project was working fine, but suddenly one day it could not build at all. The error message is simply "The operation could not be completed. Unspecified error".
I also noticed that all my local files, such function block files, would have yellow triangle symbols with an exclamation mark next to them.
If I wanted to start a new project, I would get an exception on _3S.CoDeSys.UserManagement.UserAuthentication.