@arubislander Thanks for your reply.
Actuallay I asked him before. If I understood him right it's not up to him. The decision is made somewhere else. Therefore I asked here hoping the powers that be are reading along 👓 🤞 .
Ok, the fastboot -w command did definitely work for me... but who knows.
There is another command fastboot format:ext4 userdata which should also work, but I have had the impression, that the -w command made my device boot more reliable...
Perhaps "-w" does work only after formatting with ext4?