Update!
Network connection is now working again with original RTL8102 chip!
But, in my search for errors I tested every possible thing I could think of and sadly did a factory reset on the machine.
Now we get error code 0101, Limit Position Initialize in the Service Mode has not been done.
I have follwed the steps in the service manual several times but get an error message at step 4, Position invalid out of range.
I have checked the limit sensor with the "Sensor check" in the service menu and I can see that the star "*" is present under letter "O" when the head is near the limit sensor.
That is were we are right now.