Here's the stack trace:
#0 [internal function]: XenForo_Application::handlePhpError(2, 'json_encode(): ...', '/var/www/librar...', 188, Array)
#1 /var/www/library/XenForo/ViewRenderer/Json.php(188): json_encode(Array)
#2 /var/www/library/XenForo/ViewPublic/Conversation/ViewNewMessages.php(23): XenForo_ViewRenderer_Json::jsonEncodeForOutput(Array)
#3 /var/www/library/XenForo/ViewRenderer/Abstract.php(227): XenForo_ViewPublic_Conversation_ViewNewMessages->renderJson()
#4 /var/www/library/XenForo/ViewRenderer/Json.php(88): XenForo_ViewRenderer_Abstract->renderViewObject('XenForo_ViewPub...', 'Json', Array, 'conversation_vi...')
#5 /var/www/library/XenForo/FrontController.php(607): XenForo_ViewRenderer_Json->renderView('XenForo_ViewPub...', Array, 'conversation_vi...', NULL)
#6 /var/www/library/XenForo/FrontController.php(158): XenForo_FrontController->renderView(Object(XenForo_ControllerResponse_View), Object(XenForo_ViewRenderer_Json), Array)
#7 /var/www/index.php(13): XenForo_FrontController->run()
#8 {main}