Since you have just updated from QB2012 there may be other issues to consider ..

The newer versions of QB desktop are tuned to newer versions of Windows - if you are still using the old PCs and/or if those PCs are running different versions of Windows then you can get some serious grief in making and holding network connections between users.  Then when an active user loses their network connection without logging off first, QB may appear to think they are still logged in when they try to reconnect.

The best setup is to have all users plus any data host all using the same version of windows - for QB 2017 I think that should be Windows10.  You may seem to be able to get it working on older platforms, but you then run into reliably problems as you have described.