In Windows Explorer, attempting to delete content files from the content drive fails. It says the user doesn't have permission to delete the file(s).
Attempting to change the file's security attributes fails, again complaining the user doesn't have permissions.
It can happen on more than one file. The file(s) used to be accessible and work fine not too long ago.
The files are most likely opened remotely through shared folders/sessions. If one of the remote sessions has deleted the file(s), it's possible the file(s) has been cued for deletion which would prevent the local user from accessing it.
In Computer Management, close all opened sessions, close all remotely-opened files. Stop all shares.
This will allow the local user full access to the local content drive. Please note that files that may have been cued for deletion remotely, which were previously visible but unaccessible, will no longer show up in Windows Explorer.
Now that the situation is under control, shares can be reenabled.
Users need to be careful when allowing shared access to their server's content drive, as it may result in undesired side-effects, as seen here.
Other negative side-effects include performance loss that can lead to skipping frames on playback and encodes.