Maybe, I’ve got it.
Until now, I used the link builded in ownCloud for the calendar-address of my oC-server.
With the hints of this post https://central.owncloud.org/t/ios-12-calendar-ios-stock-via-caldav/15648/38?u=hellweg-116 I got a connection to my oC and the iPad-calendar filled up.
These two points seemed to be important:
The line with “Server” needs --> server-address.de/remote.php/dav/principals/users/my-username/
For Description, enter BHICAL
After that a littlebit back, forward and save - then it works. 