Attempting to respond to a Roundcube Calendar Invitation will return with the following error to the Roundcube UI:
Application Error (500): No driver found for calendar "".
The cause of the error is commonly caused by missing tables in the 'roundcube' database.
Re-import the Roundcube initial SQL files to recreate the missing tables. Existing data will remain intact.
mysql roundcube < /usr/local/cpanel/base/3rdparty/roundcube/plugins/calendar/drivers/database/SQL/mysql.initial.sql
mysql roundcube < /usr/local/cpanel/base/3rdparty/roundcube/plugins/calendar/drivers/ical/SQL/mysql.initial.sql
mysql roundcube < /usr/local/cpanel/base/3rdparty/roundcube/plugins/calendar/drivers/caldav/SQL/mysql.initial.sql