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