@phaedrux Thank you very much for your kind answer.
Mystery solved: It was a bad crimp at the heater cable. Nothing to do with tuning! Checked the fuse which was perfectly fine and the output of the Board was 24.0V. Left only the bed heater as a possible problem.
Checked the resistance between both heater cables and was already worried that the heater was just broken but after removing the cable endings and checking again, there was continuity again with slight resistance. Reconnected and worked again!
Thanks again.
Epi