Sharing the bulk of my email response here, since it may be relevant to others who are encountering a similar issue:
I tested your Athletics feed with the preview tool and the ICS Feed URL Tester on my site, and it’s working fine in both cases, and I also was able to download the ICS file by loading the URL directly in my browser.
But… I did notice that it took quite a long time in all three cases, and I think that gets to the heart of the issue. This feed file is pretty large, and I see it has a huge number of events going back as far as 2009.
Unfortunately, old events never get cleared out of the feed (some sources do that, but Google doesn’t), and there’s no way to just request a portion of the feed from the source, so each time the feed is requested, it’s downloading all of that data, most of which is not needed.
I’m not sure if it’s feasible for you to do this, but if you could go in and delete all of the years-old events from the Athletics calendar, it would probably improve the performance. That would really be the “best” solution, but it could be very time-consuming and tedious.
There’s another option, but I’m not positive it will work. The feed may not be loading for you because of a memory issue, but I think it’s more likely a timeout issue. This solution would only help if it’s a memory issue…
Go to ICS Calendar > Settings and Utilities > Settings and bump up the value for Increase memory limit when rendering calendars. Set it to 512 MB to start, and then adjust further if needed. Be sure to run Clear Cached Calendar Data under the Utilities tab after saving the changed setting, then see if the calendar loads.
If the issue is due to a timeout, there’s not currently a way around that, but I could add it as another setting in the next update.