I know this is an old post, but this just happened to me in FMS 13. Server log shows system (Mac OS X Server 10.8) was attempting to go to sleep. But reading about this issue on FMS docs, FMS says when it receives a request from the computer's OS to go to sleep, if there are FM connected users (and there were at least a dozen in my case), then FMS refuses the sleep request to the OS. That didn't happen today. It forced all of us using FM13 Pro and FM13 Advanced off the server and the FMS was suspended and flushed the cache.
NOTE: Today is the first day our company is using FMS 13 (we migrated from FMS 12 last night), so seeing the FMS 13 force every one off the server at 10:55 am this morning was very disconcerting. I did have a backup scheduled for 11 am, but FMS 13 shouldn't force everyone off to do a backup, right!? I got the exact same error message as the OP (from a year and a half ago) as shown above.
I had a similar situation today. I hope that the problem was in the Windows Power Management. My server was set to "Go to Sleep after 30mins". I changed this to "Never" and so far I have been accessing the server remotely since I changed the setting at 1600.