Moderator: igrr
I've flashed another NodeMCU with exactly the same file, and with this, everything works fine! I also tried to reflash the original one with a standard Firmware and than reflash it with my custom one, but it still fails.
So maybe there is a hardware problem with this Board I have got. Really strange problem, but if it's only a hardware problem I am ok with buying a new one.
Thanks for all your help!