I am having the same issue, when i join a server it just shows the players and brings me back to arma 2 oa main menu. I thought it might be the arma 2 verson and when i click on View change log it says - File system error (12029) and then a ftp link to the arma2 112555 beta. (my arma 2 version is 125548 and it says its up to date).