@chrishamm @Phaedrux
Thank you very much for your inputs. I first flashed the MB6HC to the newest firmware version using Bossa but this did not resolve the issue. Indeed it was a problem with the config.txt file. By changing dtoverlay=vc4-kms-v3d to dtoverlay=vc4-fkms-v3d I was able to get the system running. Have a nice evening!

Have you disconnectede everything from the Duet except USB?

Are any components getting hot? In particular, check the SD card, W5500 chip, and main processor.