Unable to subscribe to external calendar using iCal

Unable to subscribe to external calendar using iCal

I want to subscribe to an external calendar using a URL that links to an iCal ics file.

(1) In the 'new' version of Calendar, the settings dialog is not available. There is a link in settings which should open the dialog, but nothing happens when you click it.
(2) In the 'old' version of Calendar I was able to enter the URL of the external calendar. I got the message 'Calendar subscribed sucessfully'. But the calendar is not displayed. All I can see is my personal calendar.
I have tested the iCal ics file with other applications and it works fine. It also validates without errors at https://icalendar.org/validator.htm.

Can you please help.