H.264 out of Compressor fails: 'Codec Not Installed'

Been having a lot of trouble getting Compressor (v2.0.1) to output H.264 files all of a sudden. I setup a batch, the first one will compressed fine, then the next 4 went about halfway through and then failed with an error message 'codec not installed'.

I then tried to do one file at a time, tried rebooting, tried a file systems check, tried trashing the compressor.plist preferences, all to no avail. H.264 is no longer working.

The one movie that I successfully encoded plays fine, as have all the other movies I've done with H.264 over the months...

Any ideas what I need to do to fix this?