Screen looks like garbage when using full screen on external monitor

My Minecraft version is 1.17.2

Minecraft looks like garbage - just lines that moves accross the screen, when I try to run it at full resolution in full screen on my external monitor 1680*1050, both in a full screen window and in true full screen. Please see attached screenshot.

It works fine in a smaller window on that screen, as well as in full screen on the internal 

laptop screen at 1980x1200 in both 100% and 125% scaling.

The computer is a Dell Lattitude E5570 with Windows 10 build 19043.1052.