We recently started seeing this error on both our 2.1 and 2.2 installation. It seems sometime in the last week Google started *requiring* validating and activating your domain for use with Google Apps API, wheras previosly this was optional. The Google Apps API is how the Google Docs repository plugin communicates with Google.
Stephen, there are steps here if you haven't found them already:
Here is general information from Google ( helpful for clarifying what your target prefix should be ):
We started evaluating Apps for education recently and thought this might have triggered the strict requirement. It's interesting you are seeing this error in the same timeframe as well.
I'm also interested in the roadmap for the Google Docs repository in moodle, since the AuthSub mechanism is now deprecated:
I'll have to check and see if this is something that is being actively maintained ( updated to OAuth2? )