Honestly, I don't know for sure if this is a bug, or a setup issue, but I used the zen-cart installer provided in the download to install this store, so I'm not certain it is a setup problem.
BACKGROUND: I have run into exactly the same problem setting up both zen-cart AND setting up the original osCommerce that zen-cart is supposedly based upon. I can tell you that when I first installed osCommerce, I kept getting the "too many users" mySQL error, but between errors when I could get connected myself, the cart worked very well. When I brought the mySQL error problem up with my webhost, they said I had to purchase an SSL certificate to secure the site, which I did.
Ever since installing the SSL certificate, I could not get the osCommerce cart to function. I asked a lot of questions, and read a lot of suggestions, and nothing worked. Then, I heard about zen-cart, and about how it is based on the old osCommerce code, but is supposed to be more user-friendly. So, I installed it, using the installer script provided with the zen-cart download, and BINGO, I have exactly the same problem...
...you select an item, add it to the cart, and the script tells you that the cart is empty. When I check on the admin side, none of the selected items show up in an order, so the data is definitely not being passed through. I am pretty much dead in the water until I can get this fixed, and I'm under pressure to get it done.
In a separate (I THINK) issue on the admin side, whenever I try to update a record, for instance changing a value for a given field from "true" to "false," instead of updating the record upon submission, it erases it, replacing it with an empty field.
I sure could use some help with this. Is there someone out there who can walk me through the possibilities and give me some suggestions?