With calendar 3.2.19 and the libcalendaring 3.2.19 and sabre/vobject 3.3.3 installed I get:
PHP Fatal error: Uncaught Error: Call to undefined method Sabre\VObject\Component::create() in /usr/share/roundcube/plugins/libcalendaring/libvcalendar.php:921
If I remove sabre/vobject 3.3.3 and I think it then falls back to using the included libcalendaring/lib/Sabre/Vobject 2.1.3 I don't see the fatal error. But it does not email participants about an event as expected. Only emails the first participant.
i have got the same too, install with iredmail and ubuntu 16, rouncube 1.3.8 show this error
this erros page isn't working is currently unable to handle this request. HTTP ERROR 500
from maillog this
mail roundcube: PHP Fatal error: Uncaught Error: Class 'kolab_attachments_handler' not found in /opt/www/roundcubemail-1.3.8/plugins/calendar/lib/calendar_ui.php:103#012Stack trace:#012#0 /opt/www/roundcubemail-1.3.8/plugins/calendar/calendar.php(313): calendar_ui->init_templates()#012#1 /opt/www/roundcubemail-1.3.8/program/lib/Roundcube/rcube_plugin_api.php(493): calendar->calendar_view()#012#2 /opt/www/roundcubemail-1.3.8/index.php(295): rcube_plugin_api->exec_action('calendar.index')#012#3 {main}#012 thrown in /opt/www/roundcubemail-1.3.8/plugins/calendar/lib/calendar_ui.php on line 103