[PS 1.6] Manual order price changing wrong behaviour, headers already sent

[RabbitZzZ]

Hello,

we have a strange problem on Prestashop 1.6.1.10 when adding manual orders and changing the price of a product in the cart.
We're based in Germany and have setup the taxes correctly (taxes added for all EU countries, all other countries have no taxes, based on delivery address).

Now when the customer is based outside the EU when changing the price of a product it automatically adds the amount of taxes for the base country. The information is shown that the prices are without taxes. In the summary it works alright and no taxes are indicated.

If the customer is based in the EU you can change the price of the product to any amount and it takes it as entered as the price without taxes and in the summary the right taxes are added.

In my phperror.log I get a lot of those entries (like for every change made in the form of the manual order page):

PHP Warning:  Cannot modify header information - headers already sent by (output started at [...]/controllers/admin/AdminCartsController.php:652) in [...]/classes/controller/AdminController.php on line 1819
PHP Warning:  Cannot modify header information - headers already sent by (output started at [...]/controllers/admin/AdminCartsController.php:502) in [...]/classes/controller/AdminController.php on line 1819
PHP Warning:  Cannot modify header information - headers already sent by (output started at [...]/controllers/admin/AdminCartsController.php:481) in [...]/classes/controller/AdminController.php on line 1819
[...]PHP Warning:  Cannot modify header information - headers already sent by (output started at [...]/controllers/admin/AdminCartsController.php:844) in [...]/classes/controller/AdminController.php on line 1819

In AdminCartsController.php those are lines with "echo Tools::jsonEncode" at the end of ajax functions.

In AdminController.php it's the line saying 

 header('Cache-Control: no-store, no-cache');

in the initHeader() function.

I suppose this has something to do with this.
Does anybody know the problem and could help? Thanks!