It can happen that the Windows Offline File Cache becomes corrupt. A simple solution that I used to do was just have it reset the cache by setting the registry key HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\CSC\FormatDatabase to 1.

Although this works, it removes all offline data and requires a full sync of all offline files which can take time.

In my case the FormatDatabase didn’t work for some reason and I was also not too happy to wait again a long time for all data to sync just because a single file had become corrupt.
The solution is to remove the corrupt file from the cache. This cache is stored at C:\Windows\CSC but is only accessible by the SYSTEM account. One way to access it is to reset the permissions but that also requires a restore of the original permissions in order for Windows Offline Files to continue to work correct.
Another option is to simply launch a CMD prompt under the SYSTEM account using PSExec.exe from Sysinternals.

PSexec -s -i cmd.exe

This starts a cmd.exe under SYSTEM permissions and makes it interactive. Next step is to navigate to the file C:\Windows\CSC\v2.0.6\namespace\UNC\Path\To\The\File.txt and remove it with a simple delete. Let Windows Offline Files sync the missing file and the Offline Files are all in sync and healthy again.