Thread:Extension talk:GoogleAppsAuthentification/403 Forbidden error upon return/reply (2)

For anyone who wants to know, I fixed the problem.

It was an issue with the Apache module mod_security. It prevents from urls to appear in the query string, so parameters like "http://..." cause an error. More info: http://drupal.org/node/576270 http://ericbae.com/2011/06/22/solving-openid-redirect-error/

In my case, I asked my hosting provider (I'm on a shared hosting) to add a rule and it started working just fine.