Read all posts and still didn't find a solution. Tried to verify cache, reinstall game, port forward & format my pc. This happens to every single server. (sorry for bad english, not native language)
But what ports should I open? Already tried 2303 2302 2305 & 8766 all of them TCP/UDP
Have you checked biki? https://community.bistudio.com/wiki/Arma_3_Dedicated_Server#Port_Forwarding
The message indicates lost connection to the server, it is either your network card or firewall, especially if this happens with every server.
I don't know about the network card, but the firewall is disabled. Beside that, i can't understand why it only happens with Arma III, since i can play all my other games without problems.
I've related several similar tickets to this ticket, see if you can get some ideas from those.