Unable to set DAVc calendar as default

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Steps to reproduce

1. Connect to an external CalDav provider through Dav Connector
2. Enable a calendar in the list (let's say "Calendar 1")
3. Open Calendar settings
4. Set "Calendar 1" as default for new events

Expected behaviour

The calendar should be set as default

Actual behaviour

The PROPPATCH request returns an error:

<s:exception>Sabre\DAV\Exception</s:exception>
<s:message>Property "{urn:ietf:params:xml:ns:caldav}schedule-default-calendar-URL" has an invalid value: No such calendar</s:message>

Server configuration

Web server: Apache

Database: PostgreSQL

PHP version: 8.3

Nextcloud version: 32.0.9

Start with the calendar settings flow in Dav Connector and reproduce the PROPPATCH request for an external CalDav calendar. Trace why the schedule-default-calendar-URL value is rejected as “No such calendar”; done means Calendar 1 can be selected as the default for new events without the error.