Also "Invalid mode 800x600 (-nan)" seems to be originating from Xdcv as you can see it in xrandr output (800x600 0.00Hz). Xdcv is reporting a bogus refresh rate but also Xdcv accepts that bogus refresh rate in manual mode changes using the xrandr command. So it looks like mutter needs to be loosened to accept zero-Hz.
Also "Invalid mode 800x600 (-nan)" seems to be originating from Xdcv as you can see it in xrandr output (800x600 0.00Hz). Xdcv is reporting a bogus refresh rate but also Xdcv accepts that bogus refresh rate in manual mode changes using the xrandr command. So it looks like mutter needs to be loosened to accept zero-Hz.