What is your exact Prestashop version ? Problem is appear since upgrade of your browser ?
It seems you use an outdated version who has not been patched for Ajax
This problem is fixed since 1.6.1.9, if you don’t want to upgrade your PrestaShop, you can apply the fix yourself : https://github.com/PrestaShop/PrestaShop/pull/6749/commits/73fd8dbed9f413a70f7d04fc4badd48f00ca501a