Appointments: "Additional calendars to check" shows empty list

Calendar version: 4.7.6
Nextcloud version: 29.0.2

Most of my events come from an external system, and Nextcloud subscribes to these to show them in my calendar.

But the appointments feature ignores these events, so I can’t use it. I thought I might have to specify the calendar under “Additional calendars to check for conflicts” but the “select” widget is greyed out; won’t list anything.