I have this at the very top of my send.php file:
ob_start();
@session_start();
//some display stuff
$_SESSION[\'id\'] = $id; //$id has a value         
        
I had the same error everything was correct like the setting the folder permissions.
It looks like an bug in php in my case because when i delete my PHPSESSID cookie it was working again so aperently something was messed up and the session got removed but the cookie was still active so php had to define the cause differently and checking first if the session file is still they and give another error and not the permission error