If you are still having problems try deleting every single cookie with the word pprune in it. One may have become corrupted - we haven't done anything to limit users and log ons.
Shut down your browser, restart it and log in again. Don't forget to tick the 'remember me' box if others don't use PPRuNe from your machine. If you have another browser to use you could see if it has a similar problem and forgets your log on.
Rob