"Codec not installed" error out of Compressor?

The last step in my workflow is to create a Quicktime master in ProRes 422 HQ, either 1440x1080 or 1920x1080 depending on the job. Then I run that file through Compressor to create a 720x480 anamorphic SD master, also in ProRes 422, with pass-thru audio.

Sometimes ? and this doesn't seem to be consistent ? Compressor fails on the job with a "codec not installed" error. I can't figure out why.

This morning I got this error, so I decided to troubleshoot a little. I tried telling Compressor instead of creating the SD master with ProRes, to create it with Uncompressed 10-bit 4:2:2. Same error: "codec not installed."

What the heck, thought I. I tried creating the SD master with the "none" codec. And it worked fine. Well, where "fine" means "it took forever and resulted in an impractically large file that subsequently had to be transcoded back down into ProRes 422 again before I could archive it, and also isn't 'none' an 8-bit format?"

Has anybody seen this error before? Any suggestions for how I might fix or get around it? I'm running Compressor 3.0.3 on 10.5.4 with Quicktime 7.5. Also, the behavior is the same whether or not I use Qmaster.

Thanks.