Group Policy Console error: The volume for a file has been externally altered so that the opened file is no longer valid

 Problem
===========
GPedit.msc error: The volume for a file has been externally altered so that the opened file is no longer valid

Cause
=======
This seems to be caused due to GPOs not being enforced properly on the server, causing a corrupted GPO file.

Solution
===========
Rename or delete those folder C:\Windows\System32\GroupPolicy\User and C:\Windows\System32\GroupPolicy\Machine

(note: GroupPolicy is a hidden folder, you may need to enable hidden folders to navigate to this path)

Run gpupdate