Hi,
I have Qubes 4.2.1 installed on an NS70, which is the same hardware + firmware as your NS50. The only difference with your setup is that I installed Qubes while my firmware version was still the former one before 2.5.0 (2.4.1)
I installed from the official QubesOS iso - not the modified Nitrokey version.
I wonder if you could maybe try downgrading the firmware, see if it installs and then bump to latest firmware. It could also be the newer version 4.2.2 of Qubes install iso that has a specific problem with the new firmware - it was still V4.2.1 install iso I used back then.