I finally resolved a long-standing issue for one user that was minor but quite annoying.
At some point the icon within Windows Explorer for DWG files stopped appearing; instead it gave the odd image when an icon is assigned but not found.
There had been many attempts to reset this using the “Open With” dialog, and the File Associations within Windows, to no avail.
Next I turned to the FileTypesMan program to see if I could reset the association. I could see that the .dwg file type was associated with AutoCAD.Drawing.20 and had an appropriate icon assigned, however the UserChoice setting was stuck on AutoCAD.Drawing.19 and whenever I tried to change this, it would provide an “Access is denied” error.
Eventually I discovered the UserChoice setting is stored in the registry here:
The permissions on that key had a special entry to Deny access for my user account; when removed I was able to use the FileTypesMan to re-associate the UserChoice with AutoCAD.Drawing.20 and the icon re-appeared.