Quote - To fix it, I had to go to the VSS folder and delete the temp.mc6 file. Then everything seemed to work okay - even though the script (vss.py) specifies the filename "temp.mc6". I don't quite get that, but it seems to be doing what it should.

No real mystery, Poser knows to search for the compressed file if it can't find the uncompressed one.
Until you deleted the MC6 file it didn't need to look for the (correct) MCZ because it found the previous file.