Problem with adding a product to the cart

[kazunarii]

When adding a product to the cart, a message appears that there are not enough products in stock, although in fact they are available.

This problem in particular is easy to cause in private mode. This problem sometimes disappears by itself when you click the add to cart button again.
I would like this problem not to occur with products that are in stock and should be added to the basket.

Steps to reproduce the behavior:Go to 'https://medpak.com.pl/swanson-ashwagandha-zen-szen-indyjski-450-mg-100-kapsulek'
Click on button 'Dodaj do koszyka (Add to cart)'
See error in right corner.
Additional information
PrestaShop version: 1.7.6.5
PHP version: 7.2.31

Has anyone had a similar problem and knows how to eliminate it?