QuickBooks: Multi-user stops working

Quickbooks multi-user fails.

Usually caused by upgrading the software, but it's never a clean upgrade. When QuickBooks upgrades, the old database manager remains installed. 

For example, under Services you'll see QuickBooks DB27 and QuickBooks DB30. 

The old DB engine needs to be disabled and the removed using regedit:

https://www.techieshelp.com/remove-a-windows-service-via-registry/