Laptop Monitor Brightness reduced to minimum and disabled after disconnection from external monitor.

Hi,

I have the following problem under Ubuntu 14.04. As soon as I disconnect my external monitor from my Laptop (Lenovo X1 Carbon), the brigthness is reduced to minimum (backlight switches off) and I can not increase it again. When I use the brightness control keys the brightness bar shows that it is set to maximum even though it clearly is not. Using the control keys still changes the brightness bar but does nothing to the brightness itself.

Upon a restart of the laptop the brightness returns to its maximum and is controllable again.

I encountered this problem under the Unity version shipped with Ubuntu 14.04, but it might happen with other desktop managers as well.

This problem did not exist when my Laptop was running under Ubuntu 13.10.