How to enable birthday calendars

How do I activate/enable the Birthday calendar?

In my calendar App I can activate it and the calendar is shown. However, it is empty. Also when I try to export it, I get only an empty file.


When I try to synchronize it, then I get an error:

php occ dav:sync-birthday-calendar wernfried

In SyncBirthdayCalendar.php line 120:

Birthday calendars are disabled

dav:sync-birthday-calendar [<user>]

I had a look into my MySQL database and it seems to be enabled:

select * 
from oc_preferences 
where userid = 'wernfried'
   and appid = 'dav'


And you have several contacts with Birthdays? Have you activated Contacts?
Do you have settings/admin/groupware