This may or may not be your problem, but I've run into similar issues with several epubs. Since I usually download my files on my Mac, I don't know whether or not this is an issue particular to the files distributed to the Mac. But what the heck... try this and see if it works....
- Make a copy of your epub and rename the epub extention to zip (you might have to do a cmd-I and change it from the info window).
- Double click the renamed file so that the MacOS unzips it. Do not use the unzip command in the Terminal application.
Use pdurrant's script from https://www.mobileread.com/forums/showthread.php?t=55681 (or the zip command from the same link) to recreate the epub.
- Try the inetepub.py script again.
It appears that there is an issue related to the zlib library, although it may be a problem in the zip file itself since the built-in de-zippifier on Windows seems to have the same issue. If you unzip the epub (which you may know is a zip file with the epub extension) with the MacOS built-in and re-zip it should (hopefully) work fine.