Hello Fritz,
I’m a relatively new user to balena but I have come across something similar with my devices. I had 2 almost identical Lenovo SE30 units but some underlying model number was slightly different. Well coincidentally enough, the manufacturer actually called out the ethernet interface names differently between these 2 devices.
So, from what I understand, this is actually not a balena issue, but it originates from the hardware itself and that requires you to modify your connection file. The connection file needs to know the correct interface name for the given device otherwise comms will not work on that port.
If you are ever unsure what the interface names are on a device, you can terminal into it from your balena dashboard and run “ip addr show” from the host os. It should list all possible interfaces and their respective names.
Disclaimer: I only use balenaCloud right now so if you are using some of the other balena offerings, the process my be slightly different to determine your interface names.