Facebook app for an endless loop trying to log in from the canvas

When I try to access my application from http://apps.facebook.com/myapp (FB canvas), it works fine. However, if I go straight from http://myapp.herokuapp.com (from the canvas), after entering my FB credentials, it gets stuck in a loop where the URL parameters “state” and “code” are constantly changing.

In the configuration of the FB application, the canvas URL and the website URL are http://myapp.herokuapp.com .

What could be the reason?