The following steps should be followed to solve the issue:
1. Temporary disable the multi user mode
Connect to the server and open the affected version of Quickbooks, go to File - Utilities - Stop posting. Have user log in and open the company file. You can enable multi user mode after they are logged in. That should take care of the error message.
2. Go to C:/ Program Data (hidden) – Intuit folder – give all the users full access to the folder
3. Go to Start menu and open Quickbooks Database Server Manager on the server and browse to This PC – Local Disc - C: \ and hit Start Scan.
4. Add QB database accounts to the local admin group
Connect to the server > right click on Start - Computer Management - Local users and Groups > Groups > Administrators --> include QB Database Accounts to the admin group.
NOTE: You can find the list of QB database accounts in "Users" group in the same path. To be able to add the accounts you will have to go to Location and change the location to the server name. Also, you will have to type the exact name including the caps to be able to add it.
Alternative workaround if the above doesn't work:
1. Close QB
2. Open Task Manager
3. Terminate QBW32.exe that's open under user
4. Re-open QB
5. File opens fine