Okay, so i tested the monitor again, on an Ubuntu 20.04, also via Displayport, on a physically other device.
This is the output of xrandr on that other device. It was a Laptop, therefore it has it buit in primary display. The monitor seems to be plugged in via DP-2
xrandr
Screen 0: minimum 320 x 200, current 3840 x 1440, maximum 16384 x 16384
LVDS-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 345mm x 194mm
1920x1080 59.98*+ 59.97 59.96 59.93 39.99
1680x1050 60.00 59.95 59.88
1600x1024 60.17
1400x1050 59.98 60.00
1600x900 59.99 59.94 59.95 59.82
1280x1024 59.95 60.02
1440x900 59.89
1400x900 59.96 59.88
1280x960 60.00 59.99
1440x810 60.00 59.97
1368x768 59.88 59.85
1360x768 59.80 59.96
1280x800 59.99 59.97 59.81 59.91
1152x864 59.97 60.00
1280x720 60.00 59.99 59.86 59.74
1024x768 60.04 60.00 59.95
960x720 60.00
928x696 60.05
896x672 60.01
1024x576 59.95 59.96 59.90 59.82
960x600 59.93 60.00
960x540 59.96 59.99 59.63 59.82
800x600 60.00 60.32 59.96 56.25
840x525 60.01 59.88
864x486 59.92 59.57
800x512 60.17
700x525 59.98
800x450 59.95 59.82
640x512 60.02
720x450 59.89
700x450 59.96 59.88
640x480 60.00 59.94 59.94
720x405 59.51 58.99
720x400 59.97
684x384 59.88 59.85
680x384 59.80 59.96
640x400 59.88 59.98 59.96
576x432 60.06
640x360 59.86 59.83 59.84 59.32
640x350 59.84
512x384 60.00
512x288 60.00 59.92
480x270 59.63 59.82
400x300 60.32 56.34
432x243 59.92 59.57
320x240 60.05
360x202 59.51 59.13
320x180 59.84 59.32
DP-1 disconnected (normal left inverted right x axis y axis)
DP-2 connected 1920x1440+1920+0 (normal left inverted right x axis y axis) 798mm x 335mm
3440x1440 59.97 + 99.98 94.99 89.96 84.96 79.98 49.99
2560x1440 59.99 59.96 59.95
1920x1440 85.00 75.00 60.00*
1856x1392 75.00 60.01
1792x1344 75.00 60.01
2048x1152 59.99 59.98 59.90 59.91
1920x1200 59.88 59.95
1920x1080 60.01 59.97 59.96 59.93
1600x1200 85.00 75.00 70.00 65.00 60.00
1680x1050 84.94 74.89 69.88 59.95 59.88
1600x1024 60.17
1400x1050 85.00 74.76 70.00 59.98
1600x900 59.99 59.94 59.95 59.82
1280x1024 85.02 75.02 60.02
1440x900 59.89
1400x900 59.96 59.88
1280x960 85.00 60.00
1440x810 60.00 59.97
1368x768 59.88 59.85
1360x768 59.80 59.96
1280x800 59.99 59.97 59.81 59.91
1152x864 100.00 85.06 85.00 75.00 75.00 70.00 60.00
1280x720 60.00 59.99 59.86 59.74
1024x768 85.00 75.05 60.04 85.00 75.03 70.07 60.00
1024x768i 86.96
960x720 85.00 75.00 60.00
928x696 75.00 60.05
896x672 75.05 60.01
1024x576 59.95 59.96 59.90 59.82
960x600 59.93 60.00
832x624 74.55
960x540 59.96 59.99 59.63 59.82
800x600 85.00 75.00 70.00 65.00 60.00 85.14 72.19 75.00 60.32 56.25
840x525 85.02 74.96 69.88 60.01 59.88
864x486 59.92 59.57
800x512 60.17
700x525 85.08 74.76 70.06 59.98
800x450 59.95 59.82
640x512 85.02 75.02 60.02
720x450 59.89
700x450 59.96 59.88
640x480 85.09 60.00 85.01 72.81 75.00 59.94
720x405 59.51 58.99
720x400 85.04
684x384 59.88 59.85
680x384 59.80 59.96
640x400 59.88 59.98 85.08
576x432 100.11 85.15 85.09 75.00 75.00 70.00 60.06
640x360 59.86 59.83 59.84 59.32
640x350 85.08
512x384 85.00 75.03 70.07 60.00
512x384i 87.06
512x288 60.00 59.92
416x312 74.66
480x270 59.63 59.82
400x300 85.27 72.19 75.12 60.32 56.34
432x243 59.92 59.57
320x240 85.18 72.81 75.00 60.05
360x202 59.51 59.13
360x200 85.04
320x200 85.27
320x180 59.84 59.32
320x175 85.27
eDP-1 disconnected (normal left inverted right x axis y axis)
DP-3 disconnected (normal left inverted right x axis y axis)
VGA-1 disconnected (normal left inverted right x axis y axis)
Looks promising, right? Lots of options to choose. And yes, i can choose from the GUI even, but unfortunately the screen stays also black. Could it have something to do with how linux speaks to the monitor, is that in anyway different from other OS?
Edit #1
In further research of the problem. i also booted now with an manjaro gnome usb stick on my original machine, to see what would happen.
xrandr
Screen 0: minimum 16 x 16, current 3440 x 1440, maximum 32767 x 32767
XWAYLAND0 connected primary 3440x1440+0+0 (normal left inverted right x axis y axis) 800mm x 340mm
3440x1440 59.94*+
1920x1440 59.90
1600x1200 59.87
1440x1080 59.87
1400x1050 59.86
1280x1024 59.89
1280x960 59.94
1152x864 59.78
1024x768 59.92
800x600 59.86
640x480 59.38
320x240 59.52
1920x1200 59.88
1680x1050 59.95
1440x900 59.89
1280x800 59.81
720x480 59.71
640x400 59.20
320x200 58.96
2560x1440 59.91
2048x1152 59.90
1920x1080 59.96
1600x900 59.95
1368x768 59.88
1280x720 59.86
1024x576 59.90
864x486 59.92
720x400 59.55
640x350 59.77
1920x1080_60.00 (0x266) 173.000MHz -HSync +VSync
h: width 1920 start 2048 end 2248 total 2576 skew 0 clock 67.16KHz
v: height 1080 start 1083 end 1088 total 1120 clock 59.96Hz
This is the result of xrandr of the USB Stick. It shows the options there, but i can’t choose any different resolution from the known only one. Unfortunately i forgot to safe what the error message was, when i tried to set it to 1920x1080 via CLI, i will append that in a second.
Edit #2
The promised appendix
xrandr --output XWAYLAND0 --mode 1920x1080
X Error of failed request: BadMatch (invalid parameter attributes)
Major opcode of failed request: 140 (RANDR)
Minor opcode of failed request: 7 (RRSetScreenSize)
Serial number of failed request: 22
Current serial number in output stream: 23
I don’t know if this helps, i would really wish though.
What was interesting to me, that Gnome correctly identified the monitor model (Acer Predator X34) and even load some color correction file, or something related for it. This means to me that the OS can more or less correctly identify the screen. That’s something at least.