Coupon error - Restricted access

Opencart version 1.5.5.1
I set a coupon code in admin panel (%, no login users, only 2 products with a total amount (product A + product B), when I'm going to insert the coupon code in cart page, exit this error: "Restricted access"
And then I have to delete the cookies of Safari browser to return to my homepage

Anyone can help me?

A restricted access message can normally appear if you're using OpenCart under another platform (such as Joomla, for instance) or can also occur when your .htaccess file does not populate its protection throughout the tree properly / recursively. If those two possibility does not involve your case, then possibly an installed contribution which forces restrictions on specific actions from the admin coupons.

The problem is in the language file you had to go to /catalog/language/italian/total/coupon.php

You have to remove this parts:
// No Permission
defined('_JEXEC') or die('Restricted access');

Close, save and update the file in your website and now all works