@gohan Thanks, as per your advice, I soldered 2 100 Ohm resistors between pin 1 and 2 and between pin 3 and 6 on the backside of the boards. I did this for the Ethernet module of the Sensebender Gateway as well as for the Arduino Ethernet shield on the Uno. And indeed both gateways are now perfectly working on the Ethernet Switch. MySensors forum works!! Issue solved within 24 hours.
I Am Very Grateful, @gohan
Thank you for the hints. After have been looking further on the internet involving the compile problem and found the solution, that worked for me.link text.
I inform you that it is Mac OSX python problem in the installed python version on my Mac, a broken link to the python executable.
I simply installed the newest version 3.8.2 Python on my laptop Mac and the problem disappeared.
So, everything is working now with all the latest firmwares (see above).