Rare JApplication Errror in Admin login page

[newjoomlafan]

I tried everything and this error is rare on google. Please see error in the admin section at www.kirtanmandalis.net/administrator

Fatal error: JApplication::getUserState() [<a href='japplication.getuserstate'>japplication.getuserstate</a>]: The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "Joomla\Registry\Registry" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition in/home/content/38/7965338/html/libraries/legacy/application/application.php on line 528

I would be grateful if you can help me with this error.

[newjoomlafan]

its works fine after i add www in front of the web address
but doesn't work with kirtanmandalis.net/administrator?

[jkaka]

i have the same problem but now am getting this error after restarting my computer.

Fatal error: Call to a member function isAdmin() on a non-object in C:\xampp\htdocs\kidec\plugins\authentication\cookie\cookie.php on line 53

Please help thanks