External screen connected via VGA goes black on Toshiba Tecra A8

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http://askubuntu.com – I've installed Ubuntu 12.04 on my Toshiba Tecra A8 laptop. It has Intel 945GM graphics. All my hardware components work fine except for the external monitor. When I plug in the monitor cable to the external VGA port, Ubuntu detects it, but the screen goes black and I don't get anything displayed – both on the LCD and the external monitor. How can I solve this? (HowTos)