Recieving "Requested JSON parse failed" while trying to "clear cache&save"

Hi,
i installed your template (also on localhost) and everything was working fine, the template and your framework is great, but from a certain moment, i have no idea when, i cant save the templates settings and by trying it i receive this error: Requested JSON parse failed. The same message appears by the "plugin" in advanced settings tab. No idea what could cause this or how to fix it. Really hope that someone can help me.

regards,

Tomas
Advise what PHP version you are using: 5.5.21
Advise what MySql version you are using: 5.5.34
Advise what Joomla or Wordpress version you are using: Joomla 3.4.1
Advise what YouGrids version you are using if applicable: YJSG v2.2.0
Advise if you made any custom modifications to original files: index.php

Are you sure your are posting in product assigned forum?: Yes

What is your CMS version?: Joomla 3.x

MySQL Version: 5.x

PHP version: 5.x

Link to installation: http://www.3d-sphere.com

Have you edited any files? If yes tell us which ones.: Yes

Is this personal or client website?: Personal