@cookyr: Thank you for your reply, but I am still unable to connect to the server. Instead of one "default calendar" I have multiple calendars stored on the server.
I tried both accessible URLs to no avail:
https://<domain>/owncloud/remote.php/caldav/calendars/<user name>/<calendar name>
https://<domain>/owncloud/remote.php/caldav/principals/<user name>/<calendar name>
Rainlender recognize "default calendar" as <calendar name>, but offers only an empty drop down list for the calendar names. The actual name of the calendars on the server are recognized and I receive the "url does not contain any calendars" error.
Using any arbitrary string as <calendar name> and ending the whole URL with "/default calendar" will also trigger the empty drop down list.