One of my clients have similar issue but different:

I need to run scan Folders in database engine to temporarily fix it  than few hours again it happends.

User 1 Logs in and its fine the company is multi user mode

User 2 Logs on Fine

On server database Manager shows both users are logged in.

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User 1 logs off 

User 2 is in multi user mode

On server database Manager shows users2 logged in.

User 3 Tries to log in  and is getting database not in multiuser mode 

User 2 check and database says its in multiuser mode.

Admin manually scan the folder in database server manager  all is fine 

User 3 Still cant log in

User 2 switches to single user mode

User2 Tries to switch to Multiuser mode (Cant h202 error)

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I have to manually scan the folder in database server manager 

User 2 Can switch to multiuser quickbooks

So user1 logged in and was in multiuser, User2 logg in and both were in multiuser , But the minute user 1 logs off all the glitches in a world is happening.