I found out that the minimum amount for each product was set to zero. Which means that in my case,
the product quantity at the product page was 0 whenever I tried to add a product to the cart - LOL.
I think it must be due to a bug in the upgrade data, or some missing fields.
Basically I just added minimum amount to all products via an SQL update, and all works.
Thanks for your reply @mootookoi 🙂