Cannot add feed from

I can access the following feed from my browser:

However, when I try to add it to Newsblur, I get the following error message:

This address does not point to an RSS feed or a website with an RSS feed.

I also tried downloading the feed with curl and I got a certificate error. They use LetsEncrypt; perhaps this is an SSL-related issue?

does not validate. it isn’t even seeing it as a feed with some problems: the validator does not see a feed.