I migrated a Zenphoto install from a LAMP server to a Windows server (CBeyond host) and the image links are now broken. You can see it on: http://thesunridgeranch.com/zenphoto/

Most of the images have two underscores in the URL instead of a forward slash.

I also tried regenerating the thumbnails, but 90% of the image thumbnails failed to be created.

This one worked: http://thesunridgeranch.com/zenphoto/index.php?album=life-at-sunridge&image=tyler2.JPG

This one did not:
http://thesunridgeranch.com/zenphoto/index.php?album=life-at-sunridge&image=october007.JPG

What is going on?

Please review the troubleshooting guide. This is the typical issue of images being bigger dimensionwise than your server is able to handle.