I guess, there are plenty of duplicates of this question, but I didn't find any to be helpful in my particular problem.

My story is as follows: I was running Xubuntu 12.04 on a Thinkpad X40 which has an Intel 855GM graphics chip. On 12.04 suspend worked flawless. Yesterday I upgraded to 14.04 and this broke suspend. The problem is like this: I close the lid, the laptop goes to sleep. After some time I open the lid, the screen goes on and if I'm fast I can even start typing in the password. However some seconds later the screen switches off again, though I can hear that the system continues working. I can still switch to any TTY, only X has no graphical output. If I kill X from TTY, it starts again, I can login and then the system works as expected. In /var/log/Xorg.0.log I see following lines:

(II) AIGLX: Suspending AIGLX clients for VT switch

(II) AIGLX: Resuming AIGLX clients after VT switch

(II) AIGLX: Suspending AIGLX clients for VT switch

which exactly fits to my observations: On lid close the screen goes off, on opening the lid it goes on and then after some seconds off again.

Any thoughts on how to prevent X from switching off the screen after waking up?

Thanks for any input.

Bug reported: https://bugs.launchpad.net/ubuntu/+source/xorg/+bug/1310038

As somebody pointed out in an answer that seems deleted now, a workaround is to switch off light locker.