recently a huge size increase happend in a folder in cPanel host in this address:
After 2 days finally solved the problem. I created a folder named "session" near public_html.
and added one line of php code to change the "session save path" to that folder :
ini_set('session.save_path',realpath(dirname($_SERVER['DOCUMENT_ROOT']) . '/../session'));
now session is working. I don't think this is a perfect way but at least it's working now.