Latency Test OK, dennoch Fehlermeldung

Of course:

hanno@cnc2:~$ ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host noprefixroute
       valid_lft forever preferred_lft forever
2: enp2s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 00:0a:cd:00:00:56 brd ff:ff:ff:ff:ff:ff
    inet 192.168.178.55/24 brd 192.168.178.255 scope global dynamic noprefixroute enp2s0
       valid_lft 855370sec preferred_lft 855370sec
3: eno1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 00:23:24:5b:b4:ac brd ff:ff:ff:ff:ff:ff
    altname enp0s25
hanno@cnc2:~$

Tonight, I thought by myself - maybe it's a stroke of fate, that just one connection in time  is working correctly. I would never surf the internet while running my router. In real operation, I will need internet just for updates, research, nothing really worth. BUt technically, I think that should work, and in your simulation it ran free of problems. So, I would go on...
Thanks for your time!
Hanno
 

So eno1 does not have an assigned IP address

What does /etc/network/interfaces contain?

cat /etc/network/interfaces
 

Well, that was at the time as I cut the MEesa eno1 connection in purpose to be in internet and coming into the forum to write my post.
Eno1 has a fixed IP in network manager:  10-10-10-10 MEsa resp. -2 for Intel Card.
The connection eno1 does not have an ip adress in that state. Here is an ip a with both states. Upper feedback is with connection active, lower lines without.

 

Content of /interfaces is commented out except the loopback:
 

Just a guess but you definitely  do _not_ want a gateway (or DNS) 
address on the 10.10.10.x range

And remove the "IPV4 addressierung........"

I have another idea. Maybe you can save your /etc/network/interfaces file and paste this one.
then open 2 tabs of terminal. In the first you run "sudo journalctl -f"
the system log and in the second "sudo systemctl restart networking.service". then we should see what's going on. Alternatively you can also use "sudo journalctl -f >log.txt". Then the output will be written straight into the log.txt file and you can post it.
the last step would be to delete the connections in the network manager first.

i found this here  forum.linuxcnc.org/9-installing-linuxcnc...esa-ethercat?start=0
It seems rod was in trouble but not sure how he fix it.