Weird problem of 0 products being added to the cart only sometimes when the customer first enters

[sylwekb]

Hello

On Prestashop 1.7.8.11, I sometimes have a strange problem with customers adding to cart. Despite the correct stock levels, after adding to the cart it is 0, when the customer backs or continues and adds the same product to the cart again, it is already correct. There are no rules, it can be Firefox, Edge or Chrome browsers. The problem, however, does not always and does not affect every client only for the first time. I cleared the Prestashop cache, modules updated. I don't have any errors in the error log and debug mode. Maybe someone had a similar problem and solved it? Attached I am sending 2 videos to show this problem. What exactly is the cause and how exactly do we fix it? Prestashop template coding? Page https://www.sweterkowyraj.pl/

 

Edited June 26 by sylwekb (see edit history)