The current input timing is not supported by the monitor display. Please change your input timing to 2048x1152@60Hz or any other monitor listed timing as per the monitor specifications.
Uh, does Ubuntu boot at all? Cause it might be possible to just Ctrl+Alt+F1 and login from tty1 or something
You don't need to change the resolution, just the refresh rate. There should be a setting for that in your monitor.
Resetting an out-of-range resolutionIf you set a resolution inappropriate for your monitor in the Screen Resolution GUI tool, you can reset it from a terminal by running $ rm ~/.config/monitors.xml