xrandr "Failed to get size of gamma for output default" with xserver-xorg-video-dummy on a headless NUC/BACE in 18.04

Question

I am trying to set up Ubuntu 18.04 on a NUC-like Gigabyte BACE. Everything works perfect. BUT: I would like to use Gnome with a screen resolution of 1680x1050.

This is what I did:

sudo apt-get install xserver-xorg-video-dummy
sudo nano /usr/share/X11/xorg.conf.d/xorg.conf

and there

Section "Device"
    Identifier  "Configured Video Device"
    Driver      "dummy"
EndSection

Section "Monitor"
    Identifier  "Configured Monitor"
    HorizSync 31.5-48.5
    VertRefresh 50-70
    Modeline "1680x1050_60.00"  146.25  1680 1784 1960 2240  1050 1053 1059 1089 -hsync +vsync
EndSection

Section "Screen"
    Identifier  "Default Screen"
    Monitor     "Configured Monitor"
    Device      "Configured Video Device"
    DefaultDepth 24
    SubSection "Display"
    Depth 24
    Modes "1680x1050_60.00"
    EndSubSection
EndSection`

But when I connect (via Anydesk) I get a screen resolution of 1360x768.

Here is the output of xrandr (the word "failed" is mentioned):

xrandr: Failed to get size of gamma for output default
Screen 0: minimum 320 x 240, current 1360 x 768, maximum 1360 x 768
default connected primary 1360x768+0+0 0mm x 0mm
   1360x768      60.00* 
   1280x720      60.00  
   1024x768      60.00  
   1024x576      60.00  
   960x540       60.00  
   800x600       60.00    56.00  
   640x480       60.00  
   684x384       60.00  
   680x384       60.00  
   640x360       60.00  
   512x384       60.00  
   512x288       60.00  
   480x270       60.00  
   400x300       60.00    56.00  
   320x240       60.00 

Furthermore, I try to set up the screen resolution manually. But

xrandr --newmode "1680x1050_60.00"  146.25  1680 1784 1960 2240  1050 1053 1059 1089 -hsync +vsync

gives me

xrandr: Failed to get size of gamma for output default

What can I do? Did I miss out anything?

Thanks, Joachim

OK

XORG troubleshooting starts with

cat /var/log/Xorg.0.log

it shows

[ 86133.223] _XSERVTransSocketUNIXCreateListener: ...SocketCreateListener() failed
[ 86133.223] _XSERVTransMakeAllCOTSServerListeners: server already running
[ 86133.223] (EE) 
Fatal server error:
[ 86133.223] (EE) Cannot establish any listening sockets - Make sure an X server isn't already running(EE) 
[ 86133.224] (EE) 
Please consult the The X.Org Foundation support 
     at http://wiki.x.org
 for help. 
[ 86133.224] (EE) Please also check the log file at "/var/log/Xorg.0.log" for additional information.
[ 86133.224] (EE) 
[ 86133.224] (EE) Server terminated with error (1). Closing log file.

XORG docs specify a gamma-value in the "Monitor Section".