Prestashop 1-Click Upgrade - JavaScript Error parseJSON

I have a fatal error while trying to update my prestashop from version 1.6.0.5 up to 1.6.0.9.

» JavaScript Error parseJSON

In older versions of prestashop this has been documented. But unfortunately not for this update. I have lost a few hours by searching in forums for that topic.

I am not a programmer, but a normal user and i need a solution, that can be fixed by myself.

Thank you in advance.