I subscribe to Planet Debian (whose NewsBlur URL is apparently https://www.newsblur.com/site/1171/planet-debian). For the past couple of days, whenever I view that feed, or the category it’s in, or All Site Stories, the story list never finishes loading, and consequently none of the stories I read ever get marked as read.
I usually browse in List view, with Feed content, Unread items only, in Oldest-first order, but I see the same behaviour in Firefox 44’s Private Browsing mode and Chrome 46’s Incognito mode.
Firefox’s Web Developer console has a message about "Blocked loading mixed active content “[http://…/reader/features?page=0&_=1447403464500"Learn More]” and Chrome’s console says “Mixed Content: The page at ‘https://www.newsblur.com/site/1171/planet-debian’ was loaded over HTTPS, but requested an insecure XMLHttpRequest endpoint 'http://…/reader/features?page=0&_=1447403626609’. This request has been blocked; the content must be served over HTTPS.”