Multiple Versions of QuickBooks
This error can occur, if you have two versions of QuickBooks on the same machine. If you have QuickBooks Enterprise 2018 and QuickBooks Enterprise 2019, the sync can fail because our tool fails to locate your company file and the proper version of QuickBooks. If you upgrade QuickBooks, then uninstall the older version.
Since some users run QuickBooks 2018 and you are running 2019, you will receive this error at times:
The application trying to connect to QuickBooks is not supported while multiple instances of QuickBooks are running
If you pull the SDK log in the error, you could see this entry:
Could not find or create an instance of QuickBooks using InstanceFinder
To remedy this issue, uninstall all versions of QuickBooks completely from your computer. Perform a fresh install of the latest version of QuickBooks available to your company. Once complete, open your web connector. Check the box to the left of Connex and click update selected.