The box is merely guarding some admin ports, so it is very lightly loaded, despite I tried to reset the counters, to get rid of any error counters that happened in the past.
In the setup intended it does Proxy ARP for some IPs, it does even "loadBalance", but merely to proxy the request. But I also remove the whole ProxyARP and LB Stuff and it still happened….
This are the counters after zeroing the counters, connecting, waiting in vain for some web content to show up, in hope, that something would show up in the output.