Calendar: Creation of new event fails on web ui for a single user

Nextcloud version (eg, 20.0.5): 29.0.0
Operating system and version (eg, Ubuntu 20.04): Centos 9 Stream
Apache or nginx version (eg, Apache 2.4.25): Apache 24.57
PHP version (eg, 7.4): 8.3.7

The issue you are facing:
One single user cannot create calendar events any more on the web ui. With the submission of the creation there’s a generic error message on the web ui and the below inserted Exception message in nextcloud.log. Event creation works for the same user, if created on other devices/applications and then synced onto my nextcloud server. Event creation also works perfectly on the web ui, if executed by any other user - both local and ldap users.

The issue has been introduced with the upgrade to version 29.0.0

My search in the support database has resulted in 1 similar finding from 2020: There the username contained special characters. This is not case here, which is why I assume, that both cases cannot be compared.

Is this the first time you’ve seen this error? (Y/N): Y

Steps to replicate it:

  1. Try to create any even in nextcloud calendar web ui - with the single effected user
  2. With the submission of the event creation, there’s a generic “creation failed” message on the screen and the below inserted error message extracted from the nextcloud.log
  3. Events creation work perfectly, if synced for example from my android smartphone or other calendar clients.
  4. Other users - both local and ldap users - can create events without problems on both the web ui and through other clients.

The output of your Nextcloud log in Admin > Logging:
According to nextcloud.log
nextcloud/3rdparty/sabre/dav/lib/CalDAV/Plugin.php throws an exception in function validateCalendar with the message: “iCalendar objects must at least have a component of type VTODO”

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
n.a.

The output of your Apache/nginx/system log in /var/log/____:
n.a.

Output errors in nextcloud.log in /var/www/ or as admin user in top right menu, filtering for errors. Use a pastebin service if necessary.

n.a.

I could solve the issue:

In th pop-up for the event creation the pre-selected calendar was “tasks” - though I don’t have a calender with that name. When changing this to any of my existing calenders, the event creation works. After having change the default calendar to one of my existing calendars in the app-settings this strange “tasks” calendar also disappeared from the list of selectable calendars.
Unfortunately I cannot reproduce how nextcloud got into this strange state. Anyway: It works now.

Best, Boris