I have a possible fix. We have a similar situation, different environment - 5 Win 10 workstations: 4 workstations and one acting as a server only, running database server manager.

Users will all be in multiuser mode all day, then shut down QB end of day and log out of windows. Next morning, the first PC logging into QB will get an error that others are logged in in Multiuser mode - to switch to MuM. Clicking the button fails with an error. Sometimes a 6000 error, but I've also seen H202 today.

Their tech support made a good point asking how they close out of the program- with the X or by File>Close Company/Logoff. They have been using the X. So they suggested using the correct method- File > Close.  I believe this will solve our problem. Hitting the X doesn't properly close the file handles on the server. It leaves them open and connected and gives the weird MuM error. Why it doesn't give an error of too many concurrent users- more than 3 theyre licensed for, idk. 

ALSO - dont let the bastards upsell you to Premium or anything else with more users. They'll ask how many users and how many PCs. The second you tell them you have 3-user license but 4 computers theyll try to tell you thats the problem - just upgrade to PREEMIIIUMMMM. Yeah right. I had a tech do that at the beginning of the call and then at the end she again tried to say the "server counts as a user"... BS! They're so full of it. I told her that it's impossible. Of all the many dozens of offices I've set up this way, never have I gotten a 3 user limit message with a server + 3 users on 3 PCs.  If you try to log in a 4th user- of course QB will stop you. So, no, the server is NOT a user. I can't believe they're still running those scams. I thought they had revamped their support a while ago. They're up to their old tricks.