Users logging in with empty cart go nowhere

The only similar thread I could find was this one: http://www.zen-cart.com/forum/showthread.php?t=111395

Essentially, the problem is that when a guest visits the site and logs in, they simply return to the main page, still as a guest. If the member has items in their cart, they can log in just fine. Also, if the guest adds something to the cart and logs in during checkout, it works fine.

It seems like a session issue, I've run the Cache Key fixer, fiddled with permissions on my cache folder, and made sure my cart was fully updated. Anything else I can try?

Thanks guys.

Sorry to bump, but are there at least general things I can check on this? I'm lost as to where to begin, and none of the related threads seem to offer any resolution.

two things I'd check

1. All the links in BOTH configure.php files.
2. All the links in the site to make sure there's not a hard-coded session ID in one of the links.

Checked the configure files, everything seems fine and they're the same form as my SSL cert (ie. they both use the www prefix)

I've searched all over for session codes attached to links, can't find anything out of the ordinary. Seems like a hardcoded session ID wouldn't display different behavior based on whether the guest has something in their cart; would it? Maybe I'm not sure how they work.