We have tested DAVx⁵ with Roundcube Plus Calendar.
The basic things seem to work, but there are many small problems on the server side, including:
all-day events end one day too early(fixed 16 Nov 2020)
- reminders (alarms) don't work correctly
These problems also occur with other clients like Thunderbird.
Base URL, username, password: see Roundcube: Creating a CalDAV Connection