In that case its a cache issue on your PC.
Invalid session error means your session has either timed out or been restarted elsewhere and can only to my knowledge be because youve either:

A) Been online longer than allowed with no activity
OR
B) You pc has told the server the above incorrectly.
OR
C) Your username is logged in elsewhere.

The second option is usually caused by slightly corrupt cookies and clearing the PF cookie will fix the problem.

Petrucci may of course tell you im talking bollox