[Users] Claws has stopped working with GMail in OAUTH2 testing mode

[sgeneris at comcast.net]

> for what it's worth ... i have had my claws/oauth2 in "testing" mode
> (basically following the faq) since claws oauth2 support was enabled.
> it needs renewing weekly, using the oauth2 option under the account
> configuration. it hadn't "broken" of late, but i just tested renewing
> it and it worked as it always has -- obtaining and storing the new
> oauth2 access token.

Exactly: I HAVE BEEN renewing it regularly for months, but when I tried the
other day it threw that wird error targeted at Claws developers.