Each time I log in, kalarm pops up saying: "Erinnerungen können weder erstellt noch aktualisiert werden, da kein beschreibbarer aktiver Kalender für Erinnerungen eingeschaltet ist. Um das zu ändern, benutzen Sie Ansicht -> Kalender anzeigen, um den Status von Kalendern zu überprüfen oder zu ändern." (reminders can be neither created nor updated, because no writable calendar is active. To change this, please use the view "Show calendars to verify and change the status of calendars" I have no idea, where this view is to be found. Right clicking on the kalarm icon in the notification area allows me to launch a settings dialogue, but there is nothing in this dialogue to change or modify calendars shown. In korganizer, they are switched on. All the calendars I use are from NextCloud via caldav.
ok, found it now. But it seems kalarmd does not use the calendars as korganizer, to be specific, I don't find the caldav calenders I use in korganizer. Why anyway is there a separate programme for reminders when there is already a calendaring application? This is confusing.
KAlarm is a separate specialist program which has many more options than KOrganizer does for reminders and other prescheduled actions. While KAlarm uses the same calendar file format as KOrganizer, in order to perform all its functions it defines various additional data items in its calendars which won't be understood (and will just be ignored) by KOrganizer. When using KAlarm, it needs to use its own calendars since its data may be lost if using other types.