Thanks for helping!
UEFI boot page shows that secure boot is disabled.
Actually the header of the menue says BIOS Setup Utility and not UEFI but Windows says that UEFI is used , so I followed the guide for UEFI application.
I did not disable this right from the start since in the guide the paragraph started with
so I thought this is just an option that I don’t need since I don’t plan to read/write in the Windows partition.
Anyway I now set in CMD as administrator
powercfg /h off
This made the existing installation boot once, but not a second time.
I re-installed (with formatting hard dives) Manjaro but the problem is still the same. The initial start is ok, second start fails.
What can I do?