Perhaps I was not clear.
The problem is with the maximum per order, not the maximum per user.
Let’s assume that the maximum quantity value is 2 for a product.
The steps are:
1. Customer adds 2 products to his cart. No more than that, the quantity field does not allow more than 2.
2. The buyer stays on the product page and can put 2 products in the cart again. No error message is displayed, in fact the message says that 2 products have been added to the cart.
3. However, when viewing the cart, only 2 products are in the basket, which is correct.
The problem is with the message displayed in step 2. The correct solution would be for the plugin to check how many of the given product is in the cart. It then subtracts this value from the maximum amount that can be purchased and only allows the remaining value to be added to the cart.