Thanks a lot for your effort and kind care.
well, no.
No, that’s not the issue. I can prove now.
All you need is a miracle.
Changing resolution and other settings was in vain, as expected.
All the time there was 30,0 Hz on offer (the dock can not do better and thats fine regarding 4K-UHD).
After some weeks fiddling with the same equipment I suddenly was offered the frequency to choose two times. So there suddenly was 30,0 and 30,0 Hz. Exactly the same, right.
I bravely chose the other one.
And it works.
So all the equipment was capable all the time.
You can not blame the NEC monitor: Too expensive to fail.
Neither the Librem5 (for the same reason).
Probably there was a slight misunderstanding on the frequencies second decimal place. I will follow up what experience is more common. The dark or the coloured monitor.
However.
Reminds me: In the eighties my Hyundai monitor experienced sync issues, too. I had to overheat it before use.
Librem5 rather swallows miracles … 