Sorry about that - don't want to alarm anyone...

I don't think there is any correlation between the crashes and PCTools.
I reinstalled Ami Pro, and ran compress again, with no problems.
I think problems may have been related to filling my disk until it had
0 bytes (!) left while FTPing, although I would think it wouldn't
matter since Ami Pro wasn't running at the time, and I made space available
immediately anyways.
But, whatever, the cause, it is working now.