I've been trying to deal a some version incompatibilities between version 6 and 8.5. Have you made sure that all the Crytal dlls have been upgraded to 9? I seem to recall reading that there were major changes done somewhere around version 7, but I'm not positive.
Almost all the reports I deal with (about 50) were created in version 6, most of the client computers (about 50-60) have version 6 dlls on their computers. I had a similar problem when trying to add another column to a version 6 report (which by saving it made it into a version 8.5 report). I was never able to track down exactly what was causing the problem, but as soon as I made sure the version 8.5 dlls were on the client's machine, it worked fine.
Have you tried looking for the answer at Crystal's website?