Last month while working on my client's website, I was stucked on the shopping cart error.Seems all are fine, you can use the Add Cart but if you are you are going to try subtracting the item quantity or delete the order, it will end with an error message as shown:
|Image Source : Prestashop Forum|
After reading some forums I have figure out out the problem and got fixed easily. This bug is happening on Prestashop v1.4xxx or higher versions due to conflict on SSL Shop Domain Name and Shop Domain Name. It is not caused by Ajax, so don't disable anything or tamper the source code without trying this tips.
In your Back Office under Preferences > SEO & URLs, please check to confirm that "SSL Shop Domain Name" and "Shop Domain Name" both must have the same value entered at the bottom of the page as shown in the screenshot.
|Click the image to full view|