Forbidden access (Flooding)

Most likely, this is caused by sh404SEFs security settings. When connecting with Facebook, there's a lot of communication that goes on back and forth between your site, your users browser, and Facebook. sh404SEF can construe this as an attempt to cause trouble on your site and block it.

In sh404SEF, find the security settings and either increase the flooding window (don't remember the exact name of the setting) or disable flooding detection.

The redirect issue can be caused due to the same issue. If the communication between Facebook and your user doesn't go successfully, things can get into a wonky state. This can cause loops when trying to re-establish communication. There are some things we can help with this, but it depends on exactly what's causing it (which is unique to your site). If you continue to have problems after resolving the above, let us know and we can investigate more.

Good luck!