I myself have had a lot of problems getting in but found a fix which combines all off the methods we have found.
1.First go to your documents and delete both ArmA 2 and ArmA 2 Other Profiles
2. Go to your %Appdata% roaming
3. Change the directory to Local instead of Roaming
C:\Users\Home\AppData\Roaming
C:\Users\Home\AppData\Local
4. Go to Arma 2 OA Folder Located here
C:\Users\Home\AppData\Local\ArmA 2 OA
5.Once at ArmA 2 OA folder Delete ArmA2OA.RPT
6. Once ArmA2OA.RPT is Deleted go to MPMissionsCache Folder and Delete
__cur_mp.pbo File
7. Join the Server as you normally would and Hopefully this works for you
