@Tupolov: First off, thank you for looking into the matter.

I tried that IP 46.228.195.171, but it doesn't belong to my gamserver; instead, it's the IP of an A3 Life Server named "KeksArmy Arma 3 Server Neu!!!"

Where in my logs was this IP? This smells of technical issues with the gameserver company if this IP was maybe given out twice on that January 2nd 2017...

But, alas, I still don't know exactly what to make of the fact that I could yesterday and can today join the gameserver with the "old" IP 46.228.198.108.

On the other hand, I'm very confident that eventually you WILL find the cause of those problems - I think I told you already last time you managed to help me, but I'll gladly repeat it that this mod has a constructive and helpful dev and user community that offers support on par with if not better than that of professional game publishers!