Authentication Timeout

Hey guys. Whenever I join a server it puts me in and I run around for about 5 - 15 seconds and then it shows "Authentication timeout". I've read every single thread about this and nothing is working. I've reinstalled everything at least 3 times and I've ran steam as administrator. I've deleted the files in regedit. I do not have any antivirus blocking it. Also when I go to verify my game cache in steam it says "1 file failed to validate". When I try to run the Arma 2 OA BETA I get to the Bohemia screen and it cuts me off and says "Bad serial number given in setup".

Hey. Im Getting the Same Error ive done all the same stuff as you. but yeah it keeps saying 1 file is missing when i verify. thats for both arma 2 Operation Arrow head and Arma 2

I was getting an authentication timeout trying to play DayZ Epoch Namalsk until I updated the regular DayZ to 1.8.1-alpha. Since I wasn't trying to play the regular DayZ specifically, I wasted hours trying all sorts of other things first, like fooling around with BattleEye, reinstalling, firewalls, and antivirus.