Using rouncubemail-plugin-calendar 3.3 8.1.el7.kolab_3.5

To reproduce:

• Create event, and add an attendee
• Try to save the event (event is created but no invitations are sent)

[08-Jul-2015 10:59:56 Europe/Brussels] PHP Fatal error: Uncaught exception 'InvalidArgumentException' with message 'You must pass a \Sabre\VObject\Component, \Sabre\VObject\Property or scalar type' in /usr/share/php/Sabre/VObject/Component.php:408
Stack trace:
#0 /usr/share/roundcubemail/plugins/libcalendaring/libvcalendar.php(993): Sabre\VObject\Component->__set('DTSTAMP', Object(DateTime))
#1 /usr/share/roundcubemail/plugins/libcalendaring/libvcalendar.php(946): libvcalendar->_to_ical(Array, Object(Sabre\VObject\Component\VCalendar), Array)
#2 /usr/share/roundcubemail/plugins/libcalendaring/lib/libcalendaring_itip.php(287): libvcalendar->export(Array, 'REQUEST', false, Array)
#3 /usr/share/roundcubemail/plugins/calendar/calendar.php(2030): libcalendaring_itip->compose_itip_message(Array, 'REQUEST', true)
#4 /usr/share/roundcubemail/plugins/calendar/calendar.php(1220): calendar->notify_attendees(Array, NULL, 'new', '')
#5 /usr/share/roundcubemail/plugins/calendar/calendar.php(866): calendar->event_save_success(Array, NULL, 'new', true)
#6 [internal function]: cal in /usr/share/php/Sabre/VObject/Component.php on line 408