1.6.1.11 Ajax internal server error after change quantity in BO

[wertigocore]

Hello
I using prestashop 1.6.1.11 with default bootstrap template. For some products, even when a new product, everything works except the setting quantity. I have set manually method for adjust the amount. After the change of quantity and change focus there's​ red Ajax message with "internal server error" and quantity remain the same (i.e. 0). Where can be problem? If someone was able to help me, I'd be grateful.

Thanks Honza


Edit:
From mozzila console:

jquery-1.11.0.min.js:4 POST http://maxnakup.cz/testingmode/administrace/ajax-tab.php 500 (Internal Server Error)

send @ jquery-1.11.0.min.js:4

ajax @ jquery-1.11.0.min.js:4

ajaxCall @ products.js:1416

(anonymous function) @ products.js:1501

dispatch @ jquery-1.11.0.min.js:3

r.handle @ jquery-1.11.0.min.js:3