Double check your installation. Something must be different between your upgraded installation and a clean install of Zen Cart 1.5 (lots of possibilities - from a broken template, a bad merge, leftover files from 1.3.8, renaming files to "back them up", another add-on causing the issue, etc).
When testing (Zen Cart 1.5.x):
- Passing the products_id to the category page in a URL (without POST data) did not add a product to the shopping cart.
- In the default template products_id is only added to the form (action URL) when "Display Product Add to Cart Button = 2" and "Display Multiple Products Qty Box Status = 0".
- Using the form to add products to the shopping cart still worked after removing products_id from the generated form (action URL) in the default template.
(tested with the following: clean ZC 1.5.0, clean ZC 1.5.1, ZC 1.5.0 w/ USU, ZC 1.5.1 w/ USU).
To narrow things down, see if everything works with the stock Zen Cart template and go through the steps for
Diagnosing Obscure Issues.
The only major issue I know of in Zen Cart 1.5.x with the "back" function is caused when a 404 occurs (template issues or broken link) and when all URLs (for files not existing) are set to go to Zen Cart. This was actually caused by Zen Cart adding the "404 Page Not Found" to the page history.
The fix is posted here.
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