Hallo nochmal.
Hier zur Info:
Nach wiederholtem Versuch dann doch Erfolg 
.
Lösung:
! For vnc connection to headless remote-host:
! Find Helpfull Link: https://wiki.archlinux.org/title/Xorg
5 Monitor settings:
5.1 Manual configuration: ...
!!! For a headless configuration, the xf86-video-dummy driver is necessary!
- Install it and create a configuration file !
Test:
Try Connecting (on Client) :
=> Access using Remmina over ssh-tunnel to x11vnc (on Server).
- open ssh-tunnel:
- start remmina:
OK - Connected to Remote-Host.
(at Remmina in Terminal-window)
- use remmina: -> change option=fullscreen
! Check: >> $:xrandr -q
Screen 0: minimum 64 x 64, current 1600 x 1200, maximum 32767 x 32767
DUMMY0 connected primary 1600x1200+0+0 0mm x 0mm
OK - xrandr works.
! You can use xrandr to set different resolution (must be present in the given list):
! Try mode-change: >> $:xrandr --output DUMMY0 --mode 1920x1080
! Check: >> $:xrandr -q => 1920x1080 60.00*
OK - Mode-change works.
- Summary:
OK - For using remmina in fullscreen-mode with needed resolution.
- Vermutlich war ursprünglich die Installation des xf86-video-dummy drivers das Problem.
Also alles gut , Gruß Peter.