After I converted all my apps to the new Graph API they stopped working. All of them are FBML apps. Could the conversion be the reason for this, or is FBML no longer supported?
Facebook did something and we are all in the same basket without any notice. What you should do and what I did when I got the same problem two days ago is update the SDK - https://github.com/facebook/php-sdk, change how you login - https://github.com/facebook/php-sdk/blob/master/examples/example.php and change the get_session with get_user. It will start working again