I got it solved by a hint of Max Schukin:
Press Alt+F2 and enter:
gksudo gedit /etc/X11/xorg.conf
Scroll down to the Screen section and add the following lines:
SubSection "Display"
Depth 24
Virtual 1024 768
EndSubSection
Change 1024 768 to your resolution.
In my case the lines were already present, but for some reason it said 1600 1200
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unsupported resolution while loading ubuntu
https://bugs.launchpad.net/bugs/151817
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