Help - Quicktime freezes while screen recording

When I start a new screen recording, I get as far as starting the recording. But then the recording stops and I get an error message: cannot capture. Then Quicktime freezes and I can't exit the program without using force quit.

I have downloaded the version that is supposed to be compatiable with OS X Mavericks, QuickTime Player 7.6.6_Snowlepard, but nothing has changed.

Help!