Magento

Hello,
I meet a problem which I do not manage to solve, Then maybe you could help me to solve it.
Since I spent my site on my on-line server, I realized that the button “ add to cart” does not work correctly.
I am understandable : it is necessary me myself to put in list mode so that on the page listing products the button works. But as soon as I show the detail of a product, then as soon as I click the button “ add to cart” I have an error.
I put back magento’s base template, and there disappointment, I always have the same problem. Even when I try to modify the quantities directly in the basket, I find the same error message
What will you recommend me to do?

My configuration: Magento 1.6.2.0
My server : php/ mysql