Steps to reproduce the issue

Joomla 3.6.4 + Jomsocial 4.2.5
1. if you login to site please logout.
2. after logout, or if you view site in "private view" in my browser, youe can see this Error.

Serialization of 'JXMLElement' is not allowed

Call stack

Function Location

1 JApplicationCms->execute() /index.php:49

2 JApplicationSite->doExecute() /libraries/cms/application/cms.php:261

3 JApplicationSite->dispatch() /libraries/cms/application/site.php:230
4 JComponentHelper::renderComponent() /libraries/cms/application/site.php:191
5 JComponentHelper::executeComponent() /libraries/cms/component/helper.php:380
6 require_once() /libraries/cms/component/helper.php:405
7 CommunityBaseController->execute() /components/com_community/community.php:145
8 CommunityFrontpageController->display() /components/com_community/controllers/controller.php:326

9 CommunityView->get() /components/com_community/controllers/frontpage.php:52

10 CommunityViewFrontpage->display() /components/com_community/views/views.php:251

11 CommunityViewFrontpage->showLatestActivities() /components/com_community/views/frontpage/view.html.php:119
12 CActivities::getActivitiesByFilter() /components/com_community/views/frontpage/view.html.php:301
13 CActivities->getHTML() /components/com_community/libraries/activities.php:1494
14 CActivities->_getData() /components/com_community/libraries/activities.php:1088
15 CActivities::getActivityContent() /components/com_community/libraries/activities.php:649

16 serialize() /components/com_community/libraries/activities.php:278