I'm posting a bit of information as a warning to others who might experience this same issue.

We upgraded a DNN 8x Hotcakes instance up to DNN 9.8 Hotcakes 3.5.0 and while the site looks good and works well without asp.net errors or errors in the Admin Logs, it is doing some REALLY WEIRD things in cart actions!

In this site, links were used througout the site in other modules for "Add to Cart" buttons which used the Hotcakes URL querystring action
https://www.domainname.co...HX1138&QuickAddQty=1

[i](This type of url query string link is documented in this HC page: https://hotcakescommerce....g-Links)[/i]

Before the upgrade, they worked perfectly well. After the upgrade, the following odd actions occur:
[list]
[*]when adding a first one, everything goes correctly
[*]when adding a second item to cart, the previously added product would increase quantity from 1 to 2
[*]when adding a third item to the cart, the previously (the second one) would increase quantity from 1 to 2 and so on
[*]when deleting any of the three, the fourth one stayed at 2 and then stays at 2
[*]any product added through the quickadd query string would be subject to odd actions, but normally (buttons on detail page, etc) added products are unaffected
[/list]

Once we changed it to use the other "non" quick add query string, everything works correctly
so using the AddSku does not have that problem
https://www.domainname.co...Sku=THX1138&AddQty=1