I've got this problem as well. It occurs when I use mods. So for the first connections it's always working but when i try to change a server (doesn't matter if offical without mods or modded) or log in again to the same one i get this error. Only resolution until this moment was removing DayZ profile folder from Steam/userdata/, verifying game files, removing settings, logs and mods. Until the time I'd try to play with mods it will work well. FYI: I tried to connect to different servers with different mods addded - it's always the same.