pykolab-0.8.10-1.4.el7.kolab_16.noarch
roundcubemail-plugin-calendar-3.4-52.2.el7.kolab_16.noarch

Declining an invitation for the first event of an series in the Kolab Roundcube webfrontend causes
Server Error! (Internal Server Error)

In /var/log/roundcubemail/error.log you find
[23-Nov-2018 10:09:01 Europe/Berlin] PHP Fatal error: Call to undefined method kolab_invitation_calendar::get_recurring_events() in /usr/share/roundcubemail/plugins/calendar/drivers/kolab/kolab_driver.php on line 822

This only happens when you decline the very first event of an reoccuring appointment invitation.