I notice some people join this fine while others disconnect, what worked for me was unsubscribing from Overthrow WW2 mod alone then subscribing again making sure Arma 3 and launcher is not open, you should see it re-downloading the small Overthrow WW2 mod

"what worked for me was unsubscribing from Overthrow WW2 mod alone then subscribing again"
Shortly, all problems have it's cause.
Most of them are the different files (or extra files) for server and clients.
So, firstly, server administartor must check that all players have the same mod list - they can export it from their launchers and send it to him.
Secondly, need to check that server and players have the same version, and updated.
Thirdly, if it is, but _some_ player still can't connect, those players need to do the standard Steam procedure: unsubscribe --> clean the mod folder (\Steam\steamapps\workshop\content\107410\[find your mod folder here, for OTWW2 it is 1972510873]) --> subscribe again.
And, of course, make sure the server has BattleEye disabled.

BTW, can join you tomorrow, if you give me your mod list :)

автор малолетний петух