Adding to cart does not work on mozilla, IE, Chrome, Opera

[christou]

Hello everyone,
For two months I am unable to add the product to cart mozilla, Chrome, Opera and IE.
Indeed, I work on the 1.6.0.9 version of PrestaShop, where I made a copy of the default theme I changed my convenience. Here are my problems:
1. When you click on the button "add to cart" of a product, nothing happens
2. The back office only connects more; we are brought back to the login page
Note that initially this was not the case and when you open the site the first time a browser has not this problem. I also mentioned that it works when you are in incognito on all these browsers.
Excuse my English, I am French.
I really need your help, we lose orders because customers do not come to order.
Thank you