Thanks for the detailed information, Cementknight.
You'll have to ensure all users log out properly from QuickBooks since the remaining user will access the file remotely. In this case, run the Quick Fix my Program from the QuickBooks Tool Hub.
You can find more troubleshooting steps in this article: Fix common problems and errors with the QuickBooks Desktop Tool Hub.
Just in case you're experiencing any errors when trying to switch into multi-user mode, refer to this article to resolve the issue: How to fix Error H202 and H505.
Please get back to us after trying the steps above. We're always here to fill you in.