There was an unexpected error with the request on subscribed calendars (error 301).

I'm trying to add a new calendar subscription from a booking platform. I have successfully added the first two to my calendar and they're populating. I just have this one more to go. But I come up with the message below. The URL in question works when I bring it over to the other booking platforms. It just won't come across to my personal calendar.

"There was an unexpected error with the request on subscribed calendars (error 301)".