Yosemite: Quick Look previews are transparent

Since Yosemite i'm having this strange problem with Quick Look (giving previews of files when pressing the space bar): whenever i preview my files they are displayed transparently. Pretty irritating for images, and very inconvenient for text files (see image).

I've tried switching off the 'Reduce transparency' option in the Accessibility preference pane, but to no avail.

Does anyone know if this is a bug in Yosemite or some strange setting i can change?

Solution 1:

For those still looking for this answer, a simple reboot did the trick for me.

Solution 2:

Instead of rebooting, you can simply enter "killall quicklookd" in Terminal. That fixed it.