Thanks for your help Dustatron. I was on the road the past 2 hours trying all kinds of SD-Cards and other Cameras. Conclusion: No matter which pc i used, they could not write on the SD-Cards in the camera.
Then i found an old notebook with a SD-Card reader and that did the trick! I could write the firmware and hack on the card and it all worked fine after all.
Still it surprises me, that Panasonic limit the use of sd cards like that.
Anyhow, thank you very much and thank you everybody else who tried to help.