Facebook OAuth problem? Infinite loop changing the status parameter in the url

Our application worked fine, but without any changes on our side, when we tried to enter our Facebook application outside the canvas (directly at http://example.herokuapp.com ), and after providing our user and FB password, we get stuck in an infinite loop where the state parameter in the url is constantly changing:

https://example.herokuapp.com/?state=05b40a2e4d2c1f7b4c13a026a069acc3&code=AQCql520w_5dJjHnJlojMvzSCukAMrRsjqCipdkDP3N4h4zUQH72Q79oH2EMd_tQDv-fwKTJr2LG-azBUt37bUX06r3t9QHBwb27XBQyxmgdGdR_A1flZBY_oP-L0UEhjSu1FE8v2blILC8EuEWLNikgtkmaWWwy9e93KQX00wiVj422pzHuzn-m9_jZEUiKsf3dQIwCE3V7UBHMZCD9gS0#_=_

Inside the FB canvas, it works fine. And we have a double application (test environment) that works perfectly inside and outside the canvas (the same code and configuration).

Any ideas?

thank