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:
Currently, these do not work with the
elementary OS calendar app.
To correctly load your calendars, you must copy the
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:
Tested to be working on:
elementary OS 5.1.7 Hera with
Caveat: each calendar has to be added manually