Gnome Evolution store Memos as a VJOURNAL iCalendar records.
Nextcloud use a sabre/dav framework to deal with CalDAV,
and sabre/dav provides a support to the VJOURNAL entries.
Other data entries in iCalendar is a VEVENT for the calendar records
and a VTODO for the tasks - and each Nextcloud Calendar can be limited
to store only one type of the data entries (i.e. VEVENT for birthdays)
or some combination of them.
The Nextcloud applications does not use VJOURNAL records, and regular calendars
are limited to store VNOTE and VEVENT records - Evolution can’t use these calendars
as a CalDAV source for the Memos.
But Nextcloud core (i.e. sabre/dav) could work with VJOURNAL entries without any
modification.
Valid record types for calendars are listed in the Nextcloud database,
table ‘oc_calendars’, column ‘components’:
mysql> select id,principaluri,uri,components from oc_calendars;
+----+------------------------+--------------+--------------+
| id | principaluri | uri | components |
+----+------------------------+--------------+--------------+
| 1 | principals/users/admin | personal | VEVENT,VTODO |
+----+------------------------+--------------+--------------+
To allow a VJOURNAL records into calendar set ‘components’ to ‘VEVENT,VTODO,VJOURNAL’:
mysql> update oc_calendars set components='VEVENT,VTODO,VJOURNAL' where id in (1);
After that Evolution can use this calendar as a CalDAV source for Memos.
I can’t found a way to extend calendars capabilities via Nextcloud interface,
and VJOURNAL records can’t be accessed via Calendar/Task applications,
only from Evolution Memos - but it seems to work.