I tried this with flac files (converted from alac) but I don't get that message. It may not be a bug, It must have something to do with missing/incorrect tag?
If you convert your flac to alac and still get that message when you set the rating, then there must be something in the file's tag.
You are right. I converted the flacs to alac and it still doesn't work. It must be the tag, since I now found some flac files that work without any problems. Thanks again!