Howdy Angela,
It is possible that your server does not support HTTPS urls like that one. To test this you could try downloading that iCal file to your computer, then uploading it to the iCal importer. When in the iCal importer there is a Source dropdown, that allows you to import either from a URL or upload a file. If you select “.ics file” and find the one you just downloaded, you can then see if it imports properly.
Note: Your iCal file contains over 1200 events. It will take a very fast server to import that many events. It is possible that it will timeout before importing all of them. Let me know if this happens.
Does that sound like a plan?
Cheers!
– Brook