Adding Customer Private Note In Backoffice Return Blank Page

[ovidalidodi]

In our Prestshop installation, version 1.6.1.1, we receive a blank page when the user try to add a private note on Customer Backoffice and not save the note information

 

When we click to Save Private Note the blank page URL is ajax.php

 

If I activate debug mode, it not returns any information, it is still blank page

 

Thanks in advance