Im having the same problem as Nixeh After updating my beta patch to ARMA2_OA_Build_102591 i had a pretty big problem , the game starts just fine and i can use all the buttons in the main menu except the MP button.
When i Press Multiplayer it switches to the loading screen where it says Recieving and after a few seconds the background music stops and the program locks up completely to the point where i have to either logout from my user in windows or restart the whole computer. somehow even if im out of the game it belives that im in game and gives me the game mouse icon that i cant use to click on anything but the windows startbutton and the programs in that window.
I have since tried alot of the betapatches and found out that i can run the betapatches up to and including ARMA2_OA_Build_101480 without any problems but not anyone after that one, they all give me the same error.
After alot of tinkering around and changing my firewall rules to allow everything that has anything to do with ARMA 2 and OA. I found that only way for me to get the program running and letting me past the reciving lock when i click MP is for me to completely shutdown my firewall, this works with ARMA2_OA_Build_102591 and should work for those before it aswell (haven't tested it yet though), however this is not really a good solution. This leads me to believe that something was changed in ARMA2_OA_Build_101747 that effects network comunication.
Have there been any additions in what ports should be open or have protocols changed for MP if so, what so i can reconfigure my firewall to fit the new beta patches?
the AV/Firewall that im running is ESET Smart Security 5.095.0
OS : Windows 7 Ultimate 64bit
Hardware
I7 980x
Nvidia 660 TI
16GB corsair Vengance 1600MHz