After a lot of tries and work with different adapters, cables, switches, I finally changed the IP of vmbr0. Then it worked with all network cards. I do not know why.
Also I can ensure, that there was no duplicate IP.
I have a slightly different problem, which seems to concern the network stack in general.
I have a standard vmbr0 bridge and an enp6s0 LAN interface. (just standard installation with no change, no VLANs etc.)
Since a couple of days (nothing has...