I'm on the same mission to reverse engineer how this works. A big clue is that the actual calendar plugin has a further scripts directory that needs to know about the CALDEV. To give you a clue, look at the main root Roundcube config on a working server:
```php
// backend type (database, kolab, caldav, ical)
$config['calendar_driver'] = array("database");
$config['calendar_driver_default'] = "database";
$on_ccs = file_exists('/var/cpanel/calendarserver');
if($on_ccs) {
$config['calendar_driver'][] = "caldav";
$config['calendar_driver_default'] = "caldav";
}
```
The next question arises where to find those files?
It appears on a working server. these files could be:
https://github.com/spotsh/roundcube-calendar-caldav/tree/feature_caldav/plugins/calendar/drivers/caldav Even if you solve these problems - there will still be one rather big catch - you might need a calendar server:
```bash
systemd[1]: Started Apple Calendar Server
```
So it seems due to the intricacies of getting this working, the premium providers are charging for it.
I'm trying to document an open-source journey here:
https://kb.vander.host/application-software/how-to-connect-roundcube-to-a-calendar/Please let me know if anyone has any tips! My ideal would be to get this working with Virtualmin.