Today I wanted to view some videos on Youtube again and then it happened – all I could see was a green screen.
Audio was working fine, though.
Browsing through the forums, I was able to find threads that were millions of years old, regarding Flash versions prior 11, so I couldn’t figure out at first what was causing the problem.
But I found it: Yesterday I installed a new game – Batman Arkham City. That one came with a program that is called “AMD Dual Core Optimizer”.
Apparently, this optimizer caused the video acceleration to fail in Flash videos. Deleting it and restarting my computer did the trick.