Comment 25 for bug 1870736

I have exactly same problem, I've tried both 440, and 435 Nvidia drivers.
the results of:

$ xrandr --verbose --output HDMI-0 --scale 2x2
screen 0: 6880x2880 1820x762 mm 96.00dpi
crtc 0: 3440x1440 59.97 +0+0 "HDMI-0"
xrandr: Configure crtc 0 failed
crtc 0: disable
crtc 1: disable
crtc 2: disable
crtc 3: disable
crtc 4: disable
crtc 5: disable
crtc 6: disable
screen 0: revert
crtc 0: revert
crtc 1: revert
X Error of failed request: BadValue (integer parameter out of range for operation)
  Major opcode of failed request: 140 (RANDR)
  Minor opcode of failed request: 21 (RRSetCrtcConfig)
  Value in failed request: 0x0
  Serial number of failed request: 72
  Current serial number in output stream: 72

$ xrandr --verbose --output HDMI-0 --scale 1.25x1.25
screen 0: 4300x1800 1137x476 mm 96.00dpi
crtc 0: 3440x1440 59.97 +0+0 "HDMI-0"
xrandr: Configure crtc 0 failed
crtc 0: disable
crtc 1: disable
crtc 2: disable
crtc 3: disable
crtc 4: disable
crtc 5: disable
crtc 6: disable
screen 0: revert
crtc 0: revert
crtc 1: revert
X Error of failed request: BadValue (integer parameter out of range for operation)
  Major opcode of failed request: 140 (RANDR)
  Minor opcode of failed request: 21 (RRSetCrtcConfig)
  Value in failed request: 0x0
  Serial number of failed request: 72
  Current serial number in output stream: 72