Problem with Google OAuth2

[Tom White]

I have got Facebook auth working, so my core configuration can't be too wrong, however Google OAuth2 just isn't working.

I get taken to the confirmation page ok, but when I Accept I get an exception: AuthCancelled (due to an HTTP400 being thrown).

What could be going wrong? I'm not getting any log messages, is there any way to try and log the request that's failing to find out what went wrong?