omg, I finally figured it our last night!
and in the end it was so plain & simple^^
actually my GF gave me the missing 'hint' on this one:
on the machine I've been testing last night I had a wireless adapter plugged in and she asked me why in ARMA lobby it said "LAN servers"...
cus I was on WLAN actually. and then it came to me disabling the real network adapter card in the network settings - and all was fixed up and running!
-> so on my gaming machine at home I guess it'll be the same thing - as I have 2 real network adapter cards onboard.
problem with win10 just is, that as of update 1603 the network-order has been removed and you cannot priorize a card anymore.
you will have to manually disable all of the adapters that are not in use.
I will check this on my gaming-machine asap and get back here,
ffs, this really was the problem.
so "session lost" refers to multiple enabled nics^^
just make sure you dont have any other network interface enabled, and everything is running smooth