First: Thank you so much for your patience!
Yes I did copying the iCal URL and added it to “New calendar subscription” (as it is described in your tutorial)
In the Account settings of the calendar app there is now option to add an URL. Neither is the option anywhere else…
What I could try, since you say it seems to work with google, that I create a google-calendar and add that google-account to my Apple-Calender. Is a bit of a workaround, but if it does the job…