Have you tried using a different name for that mode? I was getting this error and tried using the name “1080p” (e.g. xrandr –newmode “1080p”…) instead of the “1920×1080” name that I usually use and it worked. I’m not sure what’s causing the error, as the 1920×1080 mode isn’t even listed by xrandr, nor can it be removed via –rmmode, but creating a new mode with a different name seemed to work for me.