Can you check the permissions on your folders, or folders that contain a file that's giving you an issue? Sounds like one of the folders has the wrong permissions. We store things on a network drive too and not long ago I had issues where any folder created on one computer would cause permission problems. Nothing had changed, I'm the only one that used that computer, but somehow it started hosing up permissions when creating a folder from that machine.
Check those permissions and make sure they aren't varying.