SUPPLEMENT to the request:
after several tests with different ICS files from different sources (always without success) I compiled the ICS shortcode manually and set the parameter “mapsource” to “bing” / “openstreetmap” ([ics_calendar url="https://..." view="month" location="maplinks" mapsource="bing"]) and it worked.
So the error is in the calendar generator / ICS calendar configurator.
Here the value from the “map source” is not taken over.
I enter the error here in the forum under “Troubleshooting and Bugs” explicitly, in case other users stumble across it.