I'm having this problem when I try to joins lots of servers. It isn't just small servers it is big ones like XR-DAO and 7cav, but it will let me join ones I host.
I've read about and seen that if you copy the ArmA 2 folder into the arrowhead folder it will fix it, but I have a problem I bought combined operations from Steam, so I don't have that folder. To fix it I have tried copying all files inside my common folder from inside the arrowhead folder then making an AddOns folder inside the arrowhead folder and pasting them in that. It didn't work. I then reverted back to where I was at the start and here I am can't fix it.
Fresh install of BE
Fresh install of ArmA 2 about 4 times
Steam validate files (Checks to see if files are missing)
Made a copy of folders and tried to replicate add ons folder to make it work
Ran ArmA 2 CO from steam
Ran ArmA 2 CO from the CMD file located in ArmA 2 folder
So I have no idea how to fix this and I don't think I have much hope of getting it fixed as I have seen a few no reply threads with the same issue. If anyone knows how to fix this combined ops purchased from steam then please post!