Thankfully this happens rarely, but on occasion users are not able to open any hosted files when presenting the proper credentials.  We're using external authentication with Active Directory.  I've noticed this issue with FMServer 11-13, Mac or Windows server OS doesn't matter.  I am able to open hosted files with a local account and password, it's only externally authenticated accounts that can't be used.

It appears that the server is no longer able to communicate with the Active Directory. It happens regardless of wether there are users on the server currently or not.  The only way that I've found to cure the issue is to reboot the server.  Which isn't a solution that I care for.  We're currently running FMS13.0.5.520 on OSX 10.9.5

The FMS and OS logs don't give any indication of what could have happened.  Is there somewhere else that I can look for clues on why the communication isn't happening?  Is there any suggestions on restoring the connectivity that doesn't involve kicking users off the FM system?

The link is strictly between the OS on the FMS box and the AD, so there is nothing to do on the FMS side.

The logs on the OS may yield a clue...

I've never experienced like this and it sounds like you experience it all the time?   The only failures I have seen is when the AD is too far away from the FMS box and the request times out.  But we're talking thousands of miles and poor networking.