Can't log into albums, only main Gallery, after upgrade to 1.5.7

After upgrading, I can log in fine to the gallery at the main gallery URL (below). But when I navigate to an album or sub-album, it no longer recognizes that I'm logged in.

While at the album or sub-album level, when I click the Login link and enter a username and password, it does refresh the page but I'm still not logged in at the album level.

If I go to the main gallery level, log out, go to the album level, and log in, it still thinks I'm not logged in at the album level, but when I navigate back to the gallery level I am logged in.

I did the session test, and it passes. It does increment the number. I've also tried clearing cookies, and that doesn't make any difference either.

I've tried several different user accounts, all with administrator privileges, and the behavior is the same for all of them. Same behavior in Firefox and IE7.

Any help would be greatly appreciated. TIA.

Gallery URL (optional but very useful): http://necessaryvirtues.com/home/gallery
Gallery version: 1.5.7
Apache version: ?
PHP version (don't just say PHP 4, please): 5.2.4
Graphics Toolkit: ImageMagick
Operating system: Debian Linux
Web browser/version (if applicable): Problem occurs in Firefox 2.0.0.15 and in IE 7, same behavior.