Cannot get an Access Code for GCal in Appointments+

When setting up GCal sync with Appointments plus, I can get as far as "Generate your access code" but when I click the link, I'm given the following error:

401. That’s an error.

Error: invalid_client

The OAuth client was not found.

All steps have been verified by staff and the site has been added to the Domain Verification section correctly.

Plugin conflict test performed without success.