facebooker2 error

I am using facebooker2 gem on Ruby on Rails 2.3.8.

This works very well. However now when i try connect with a user this error is thrown.

Mogli::Client::OAuthException (Error validating access token: The session is invalid because the user logged out.):

I don´t understand why this occurs.

is the api key the api id as well?

Thanks in advance

UPDATE:

Now I can see the problem. There is a bug with sessions/cookies. If I delete the cookies I can connect correctly. However if I disconnect and then reconnect the same error is thrown

I use fb_logout helper. However how can I delete the Facebook sessions? how can I prevent/catch/rescue this error?

Thanks in advance again.