Tested with Chrome now, on chrome it works. So it seems only a firefox problem.
I switched off the extensions ( https everywhere, ublock origin and privacy badger ) but still crashes.
Had htop running on my client while crashing, no unusual increase in RAM usage while going to crash.
Rebooted XO VM, now it is ok again with firefox.
so for me the issue is closed, if somebody is bored and wants to look into some logs for bug hunting i am glad to provide them.
i also restarted the toolstack on one of the pool members ( not master ) and the "no such host" error is gone.