just wanted to tell the possible end of the story.... if anyone here ever has an simelar problem :-).
Currently it looks like that the Fan caused vibrations on the case that where transmitted to the hdd nearby.... the hdd is an SMR harddisk that seems to have problems with such vibrations.... that caused the problems on server1.
server2 had an nfs connection to a vds on server1. When IO was going to 100% because of the hdd error on server1 it caused exactly the same behavior on server2 (no idea why) even when there are no physical disk problems on this device.
Overall no XCP-NG problem, but such strange that it's hard to imagine what could cause this at the beginning :).