[ubuntu] White screen when pc locks

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http://ubuntuforums.org – Hello there, I have recently switched to ubuntu 12.04, I used to be a windows user and my teacher convinced me to use ubuntu. I love it! Only two problems. I have a Dell xps 15z and when I lock the computer using ctrl+alt+L I get a white screen and I can't do anything else then give it a hard reboot. Same thing happens in suspend (close the lit or choose suspend from the menu) only then the screen turns black instead of white. Is this a known issue? Please help. With kind regards, Wesley (Hardware)