Comment 2 for bug 1929456

As I said, the workaround worked. To help others looking for a solution and finding this bug, I will show it here for my configuration:

1.) Open a terminal

2.) Call xrandr to see, how your desired device is called. In my case, it's called "HDMI-1". If you have more than one, you may guess, but output should by clear enough.

3.) Call cvt with desired resolution and frequency. In my case it must be 2560x1080 with 30 (Hz), If you are unsure you have to try (I tried 60 Hz first - didn't work in the last step no. 6):

cvt 2560 1080 30

Output in my case is

# 2560x1080 29.97 Hz (CVT) hsync: 33.03 kHz; pclk: 106.75 MHz
Modeline "2560x1080_30.00" 106.75 2560 2640 2896 3232 1080 1083 1093 1102 -hsync +vsync

4.) Call xrandr to set new mode, copy everything after "Modeline":

xrandr --newmode "2560x1080_30.00" 106.75 2560 2640 2896 3232 1080 1083 1093 1102 -hsync +vsync

(The name in quotation marks could be changed, but no need to do so.)

5.) Call xrandr to add this mode by it's name to your desired device:

xrandr --addmode HDMI-1 2560x1080_30.00

6.) Call xrandr to set your desired device (here. HDMI-1) to the mode

xrandr --output HDMI-1 --mode 2560x1080_30.00

Instead of step 6 via terminal you could use the GUI to change the resolution, it should be there now.

Hope this may help somebody.

It is not persistent. I made a script and added it to autostart. There may be other ways, maybe a config file for xrandr?