Login as customer AND admin fails ALMOST always

Hi all, I'm a Zen Cart newbie stuck on this problem after days of tinkering and Google-fu, to no avail.

I am running Zen Cart 1.3.0, with template: Free Yellow, version 1.1, running on a Unix server at gaiahost.coop. After installing the Free Yellow theme, I changed the header image and text, and made a few changes to the template's css to affect font color alignment.

When logging in as admin, virtually every time I have the try more than once before successfully logging in. Sometimes it takes a few attempts, sometimes it takes dozens, but eventually I do get logged in.

I tried creating customer accounts, and the process completes, I get the confirmation email, as if all is well. Then I try to log in as the customer and that fails even more often than the admin login, but sometimes I can log in.

Even in the rare event that I successfully log in as a customer, when I add items to the shopping cart, then go to view the shopping cart, it is empty.

The first time my admin login failed, I cleared Firefox's browser cache, and that seemed to work. However, I quickly discovered clearing the cache was not a cure, even if it did seem to help on some attempts. Eventually I tried to log in as admin and customers on 3 different computer, each with a different OS, and about 5-6 different web browsers, with the same results.

I feel my sanity weakening as I try to figure out where to go from here. I'm sure it won't come as a shock to anyone that I am under pressure from a client to figure this out ASAP.

Sorry for the long post. Any help is much appreciated!

I am running Zen Cart 1.3.0,
That version is four years old....
I have not had that code running since the 1.3.0.2 update in June of 2006 as I suspect most others have not
Even in the rare event that I successfully log in as a custome
What happens when it fails??
Does it loop to some page?
Same with admin - there are many threads about admin login loop back to login in screen

Thanks for replying. When I try to log in as admin, the same page reloads with the user name and password fields blank, no messages or error of any kind are displayed.

When trying to log in as a customer, the same page reloads as if nothing had happened.

All threads I could find about admin login failing describe slightly different situations in which login attempts never succeed, or return errors of one kind or another. I did find a couple of threads addressing my exact admin log problem (looping back to same login page with no error), and the solution was clearing browser cache, which although occasionally helpful, mostly does not work.

There is a debug module available in the free software downloads area that shoul log the issues and point to the problem

Thanks. I ended up doing a fresh install with the latest code, and everything works. I never got to figure out what went wrong, but time won't afford me that luxury. Now I don't dare install a template for fear or breaking it again. Thanks for the help!