I have trouble using Facebook auth on my website...

I'm using the PHP Sdk and I don't understand why I can use some functions like "getLoginUrl"/"getLoginStatusUrl"/"getUser" (with right answers : the facebook session is started, i get the user's facebook ID) and when I want to use something like

$me = $facebook->api("/me/permissions");

or just

$me = $facebook->api("/me");

there's always an exception and nothing works ...

I guess it has something to do with permissions or token maybe, but I don't know what. When the user sign up on my website, I ask permissions I need with this scope : "email,user_about_me,user_location,read_friendlists,publish_stream"

A clue that the permissions seem to be ok is that when the exception occures, I ask the user to sign in again (facebook connect) and no window appears as if everything's ok, but still, the page is reloaded and the exception appears again and again...

Please help, Thank you !