There are a number of things you could try. This is an issue with Epoch, not the server, so the Epoch devs have to fix it. Until then, try one of these :
1. Delete the OAMPMissionsCache folder at C:UsersYOUR COMPUTER NAMEAppDataLocalArmA 2 OAC
2. Join another server successfully, then quit and try and join ours again.
3. At the waiting screen, hold down shift and press the minus (-) key on your numberpad. Let them both go and type f l u s h. This will flush your videoram.
4. Go to battleye.com and download the latest battleye filters. Put them into your Arma 2 : Operation Arrowhead / Battleye folder, overwriting the ones already in there.