Can't tell. Your issue report references Kunena scripts. If you give something using only core it might be easier to trace but as is there are too many variables to try and debug this. Also Registry doesn't have __get() or __isset() methods so if it is that same PHP bug it's an odd mutation to say the least.

C:\Program Files (x86)\Zend\ZendServer\data\apps\http__default__\0\kunena\3.5.0_3\plugins\kunena\joomla\access.php:260

is this code:

public function getAuthoriseActions(KunenaForumCategory $category, $userid)
    {
        $groups = (array) JAccess::getGroupsByUser($userid, true);
        $post   = array_intersect($groups, (array) $category->params->get('access_post', array(2, 6, 8)));
        $reply  = array_intersect($groups, (array) $category->params->get('access_reply', array(2, 6, 8)));

        return array('topic.create' => !empty($post), 'topic.reply' => !empty($reply), 'topic.post.reply' => !empty($reply));
    }

and that's loaded this file:
C:\Program Files (x86)\Zend\ZendServer\data\apps\http__default__\0\kunena\3.5.0_3\libraries\kunena\forum\category\category.php

To me the error looks like that $category->params is a string, not Registry object. Not sure if it is related to the above change in PHP or not.

Closing as its not joomla core

We have fixed the issue by:
Before
$instances = (array) $db->loadObjectList('id', 'KunenaForumCategory');

After fix:

        $instances = (array) $db->loadObjectList('id', 'KunenaForumCategory');

        foreach ($instances as &$instance)
        {
            $instance = new KunenaForumCategory(array('id'=>$instance->id));
            $instance->load();
        }