Facebook PHP API - sudden problem with users_isAppUser()

Question

Hello,

we have an iframe facebookapp which worked fine till yesterday. Although we didn't change anything our app suddenly stopped working.

We could identify users_isAppUser() as a problem. The method returns that the user has not added the app though he definitely has installed the app and is logged in. We could delete the try/catch part (see code below), so that the app does not get catched in a redirect loop, but the following methods do not work either:

$this->facebook->api_client->friends_get()

$this->facebook->api_client->friends_getAppUsers()

$this->facebook->api_client->call_method('facebook.users.hasAppPermission', array('ext_perm' => 'publish_stream'))

require_login() does work and we can get the facebook userid of the logged in user.

The weird thing is, that our app worked fine for a couple of weeks till yesterday. Have there been any secret changes to the API in the last days? Or any other conclusions what the problem could be? I would appreciate any tips. Thanks in advance!

$this->fbuserid = $this->facebook->require_login();

// check if user has added app,  exception gets thrown if the cookie has an invalid session_key i.e. user is not logged in
try {
    if(!$this->facebook->api_client->users_isAppUser()) {
        $this->facebook->redirect($this->facebook->get_add_url());
    }
} catch (exception $ex) {
    // clear cookies for application and redirect to login prompt
    $this->facebook->set_user(null, null);
    $this->facebook->redirect($this->configArray['appcallbackurl']);
}