Are you using anonymous anonymous? Are using Initial Data Reduction based on Section Access?
Make sure the Section Access table contains the end-users/service account name and permissions, in order to get access to the file. Failure to do so can result in one locking themselves out from the file in a no-reverse scenario.
It always help if you will give the version number you are running.
The service account that the services are running under is listed as administrator in section access. Now, with that said, being it works some of the time and a server restart fixes the issue, I would think maybe an issue with the release you are on.