I presently have 1 main album containing 63 sub-albums which each contain 100 images.
If I try to pre-cache all of the images in the main album, the operation eventually fails after roughly 50 albums. Then, if instead, I try to pre-cache individual sub-albums I get a "Object not found" error message - perhaps due to mod_rewrite? I have found that editing albums with large numbers of sub-albums is a tremendously slow and oftentimes impossible process.

Since you seem to be new to Zenphoto:

1. It is not necessary to precache all image at all. Please read this: http://www.zenphoto.org/2008/08/troubleshooting-zenphoto/#22

2. Try disableing the "visual thumb selection" in the admin options (gallery tab).

Thanks for your reply.

I need the thumbs to already exist so they are available for other applications, so the pre-cache feature is ideal for me.

As you suggested, I disabled visual thumb. Also, I reduced the number of sub-albums to 10 so that I can pre-cache 1000 images at a time from the top-level album.
This seems to be working better.

Thanks again.