Are these 1201x631 px imagers your originals? Do you include the "Scaled images" or only the "originals"?
He's including only originals, and when I test that, with a few of his PNG's, everything works normally.
If the ZIP is missing from the local directory (Ctrl-Shift-O), even though the button is generated, then it can happen some security software removes the file automatically.
I had begun to suspect this, as well.
The only other thing I can suggest is doing a complete uninstall/reinstall of jAlbum. Maybe there are some pieces of obsolete code kicking around. It will take only a few minutes.
https://jalbum.net/forum/thread.jspa?threadID=35746