Subscribe iCal - Error message

I use Nextcloud 17.0.1 and I like to subscribe to an iCal calendar.
https://info.zke-sb.de:443/WasteManagementSaarbruecken/WasteManagementServiceServlet?ApplicationName=Calendar&SubmitAction=sync&StandortID=121047584001&AboID=39337&Fra=R4;P;G&Tag=1&Zeit=9

But I get a error message:
Error requesting resource on remote server. This could possibly be related to a certificate mismatch.

I don’t know what is wrong with my installation.

Make sure that required intermediate and root certificates have been installed on your server:

grafik

OK, but how do I install these certificates at my Nextcloud