I've had this problem before and the answer was on proxy2.de's forum.. unfortunately the forum is down.. guess I should have written it down for future reference:

After installing Advanced Guestbook and setting up the table, you can view and sign the guestbook, but the default login info will get you to the first level of the admin menu, but when you try to go to Easy Admin or any of the other menu choices it dumps you back to the login screen.

The solution was simple when it happened before.. wish I remember what it was... any ideas?