Hi,
Did you update your HikaShop ?
I can see that the "access level" setting of your product is not set to all the user groups. Could it be linked to the problem ?
Also, you provided the screenshot of a product with variants. Does that mean that the element with the id 366 in the error message is the main product ? Or it's a variant ? If it's a variant, could you also provide a screenshot of the settings of the variant ?
How is the product added to the cart ? Could you provide the URL and instructions to reproduce the problem ?