Membership plugin generates Fatal error: Call to a member function get() on a non-object

Hi, on one of my network sites (www.wolfgangrathert.com) where I have the Membership plugin enabled I get a white screen showing the error message below. The error message does not show up if I am logged in as an admin. I have WP 4.0, multisite enabled, Genesis framework enabled, theme "Minimum Pro", Membership plugin 3.5.2.1 enabled only on selected sites.

Here the error message:

Fatal error: Call to a member function get() on a non-object in /srv/www/vhosts/smart-business-events.com/httpdocs/wp-content/plugins/membership/classes/Membership/Model/Rule/Categories.php on line 378

As a quick fix in deactivated the plugin, and the site was available again.

Thanks for ideas how to get the plugin working again...