We had the same problem, almost daily. What's crazy is that it wouldn't kick users out of the company file when it happened, so users could be logged in and working, but the next user who attempted to open QB would receive the error. See attached errors.  Not sure if this will help anyone but I did the following. Our setup has the file hosted on a Windows Server (without the QB software), and the QB server software running on a Windows 7 computer. 

1) Uninstalled all versions of "QB Enterprise Server XX" on the hosting server except for QB Enterprise Server 20.0 which is the version we are on. Whenever we upgraded versions, the old Enterprise Server versions would remain. They never bothered us but we uninstalled just in case.

2) File->Utilizes->"Stop Hosting Multi-User Access"  on the QB server with the software.

3) Reboot both PCs.

4) Open the DB Server Manager on the hosting computer. Go to the Ports tab and hit "renew" next to Enterprise 20.0 to get a fresh port. Do the folder scan to share the files. 

5) Open the company file on the server and then host multi-user access again. 

I want to change our setup so that the QB software is also on the computer where the file is hosted. I believe from looking at the .ND file and some of the other errors that the DB Manager couldn't tell if the file was being hosted on itself or on the server with the software.  Hope this made sense.