Thank you for your post, and I apologize for the late reply.
I am able to replicate the problem under Windows XP. This does not crash under Mac OS X 10.7.2.
Also, it doesn't matter what is the file type (Excel, comma-delimited, etc.), the script crashes when the Group By has fields that aren't included in the Sort.
I have forwarded your post along with my findings to our Development and Testing departments for review. I will keep you updated as information becomes available to me.