I periodically get this error when running OpenSync. The problem goes away after I log in to QuickBooks on the machine running OpenSync. This makes it impossible to use the OpenSync scheduler reliably. Our QuickBooks license is not expired.
I am also having the same issue. Checking the recent logs it has occurred for me on 11/14, 11/29 and now today, 12/6. I have other third party apps access QuickBooks but do not have this issue with them.
I am on OpenSync v3.0.25 and QuickBooks Enterprise 19. My logs look just like adams above.