I worked on this for days, even hired the Google API "Guru" to help me. I rewrote the rss.php page to print the feed in the format they needed, and even though the feed was perfect, it still would not work.
But when I copied that feed and those images out of the zenphoto directories and put them into directories that were not mod_rewrite, the slideshow worked.
There is something about the mod_rewrite (I suspect) that is causing the API to crash. Sometimes I could get it to work for two or three tries, and then it would stop.