Hi fabster82,
You can open the /plugins/system/t3/base-bs3/html/com_contact/contact/default.php
Line 69, replace this code:
<?php $contactLink = ContactHelperRoute::getCategoryRoute($this->contact->catid); ?>
With the following code:
<?php $contactLink = RouteHelper::getCategoryRoute($this->contact->catid); ?>
Hope this helps!