I've encountered the same problem and fixed it by specifying a direct calendar endpoint. This does not require the installation of any additional software or settings.
The two endpoints automatically generated through the online Nextcloud Settings
function within the Calendar app are in the following format:
https://example.nextcloud.com/remote.php/dav
https://example.nextcloud.com/remote.php/dav/principals/users/USERNAME
Currently, these do not work with the elementary OS
calendar app.
To correctly load your calendars, you must copy the individual private link generated by opening the drop down menu beside each calendar and clicking Copy private link
. This will give you the following endpoint format:
https://example.nextcloud.com/remote.php/dav/calendars/users/USERNAME/CALENDAR/
Tested to be working on: elementary OS 5.1.7 Hera
with Nextcloud 19.0.5
Caveat: each calendar has to be added manually