gpedit.msc) I received the following error:
This is actually one of the easiest errors to fix. If you take a look at the screenshot, you'll see that it lists the file that's causing the issue. In my case, it was
C:\Windows\PolicyDefinitions\inetres.admx. However, replacing that file with a known good copy did not resolve the issue. The solution is inside
C:\Windows\PolicyDefinitions\en-US\there is a corresponding
.admlfile. Replacing that file as well allowed me to open Group Policy with no issue.
Note that since these are system files, you will need to take ownership of them first.