Hello,
There is a typo error on the 1.6.6a link,
https://www.zenphoto.org/pages/download/
Link is
https://githu.com/zenphoto/zenphoto/archive/refs/heads/master.zip
instead of
https://github.com/zenphoto/zenphoto/archive/refs/heads/master.zip

Thanks, fixed!

You can also always get that via GitHub itself: Click the green button on the top left and choose "Download zip".