SR has lost QB connection

ShipRush is not recognizing that QuickBooks is open, so it won't get past the initial error screen. Then, when we try to exit QuickBooks, we're told that SR is still running and connected, so QB can't be closed.

Possible cause: We opened a different QB database yesterday, and when SR attempted to connect to it, we got the error message and proceeded in standalone mode. Other than that, only normal operations with no explanation for error.

Steps taken:
1) Deleted existing SR permissions/certificates in QB Integrated Applications
2) Logged into QB as Admin, launched SR ("Run as Administrator"), and added a new Certificate.
3) Started SR - failed.
4) Forced exit of QB and reloaded QB, then SR - same error.
5) Rebooted machine - SR still failed.

What's next?