Can't Login With Empty Visitor's Cart

[Amoria]

Zen Cart Version: 1.3.8
Site: http://cafeamoria.com

We are in the prelaunch phase with our Zen Cart setup and this is a bug that I have known about for quite a while, and have finally decided it needs to be dealt with. My problem is simple: if a "guest" comes to the store and tries to login using an existing account, they are redirected back to the home page (like normal) but NOT logged in. Previously, a second attempt would result in a successful login. Now, any number of attempts will result in failure. However, if the user adds 1 or more items to their cart and THEN tries to log in, the attempt is successful on the first try. In other words, if the "visitor's cart" is empty, login is impossible.

This sounds like it might be some session-handling issue, but the shopping cart full/empty trick has thrown off my troubleshooting. I'm not sure what to diagnose from here.

Halp.

[Amoria]

A few more details:

PHP: 5.2.6
MySQL: 5.0.51a
OS: Linux