Since I was involved at a later time when the problem already existed, I can't give the steps to reproduce for sure. It was told to me, that before the update to 3.0.4 of the calendar app, everything worked fine, but after the update, the calendar of one user didn't load anymore.
I verified, that with some users the calendar app just worked fine, but using one particular user resulted in endless loading of the calendar app. There was no freezing and one could change to any other app, but the calendar was not loaded and no calendar specific options (add event, ...) could be performed.

After duplicating the cloud to have a playground I managed to track down the issue:
In the mySQL DB in table 'oc_properties' there were the following entries (shortened and anonymized):

MariaDB [nextcloud]> SELECT * FROM oc_properties;

+----+---------+-------------------------------------------------------+------------------------------------------+---------------+
| id | userid | propertypath | propertyname | propertyvalue |
+----+---------+-------------------------------------------------------+------------------------------------------+---------------+
| 7 | user01 | calendars/user01/user02 | {http://owncloud.org/ns}calendar-enabled | 1 |
| 10 | user02 | calendars/user02/personal | {http://owncloud.org/ns}calendar-enabled | 1 |
| 11 | user01 | calendars/user01/personal_shared_by_user02 | {http://owncloud.org/ns}calendar-enabled | 1 |
| 12 | user01 | calendars/user01/personal_shared_by_user02 | {DAV:}resourcetype | Object |

The last line was the problem. Deleting the line with
MariaDB [nextcloud]> DELETE FROM oc_properties where id=12;
solved the problem.

Calendar app

3.0.4

Client configuration

not relevant

Server configuration

Operating system: Ubuntu 20.04.3 LTS
Web server: Apache/2.4.41
Database: mariadb Ver 15.1
PHP version: 8.0.14
Nextcloud Version: 23.0.0
Updated from an older installed version or fresh install: Updated

HTH, if anyone encounters the same issue.