Here is my PHP.INI file. Compare it with yours. 'Private Browsing' doesn't do any difference. Do all this in your localhost (your PC using XAMPP or WAMP) and let us know. I am also attaching the YITH plugin which I am using. If my PHP.INI cannot solve the problem, install a fresh WordPress and the install this plugin. See if that solves the problem and do clear the browser cache and cookies of localhost before doing this.
Note: One of the YITH files attached here is the license file which I use but it will make no difference. Put the license at the time of installation of this plugin. But it may not show in the '
License' page but it will work... at least it is working for me.
And here is the proof that it is working: