For me it also does not work, when I boot up with both screens turned
on. Try booting it up with the ThinkPad's display closed, then it should
work.
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https://bugs.launchpad.net/bugs/732050
Same here. I have a ThinkPad X201 and a Dell 24" display connected via
DisplayPort to the X200 UltraBase. I can log into Unity without a
problem but when trying to change the resolution, I get a blank screen.
But I can "solve" the problem by running this in terminal:
xrandr --output HDMI1 --mode 1
