Looks like the url got changed (the 301). Does it say the same thing in Site Settings? Make sure to reload the site before you check so you ensure you have the latest.
I tried changing the URL in the “Change Website Address” to the HTTPS one (since Mark moved the website permanently to HTTPS a month ago). NewsBlur seems to have recognized it as a new feed and tried to fetch the new feed, but no luck.
Reading the entry I linked above, I found this:
In my testing, this generates a 403 with the message “TLS SNI Required.” for clients that don’t send SNI information.
403 really isn’t the greatest status code for this semantic, so I filed a bug with Apache to make it more appropriate, and to give more flexibility in the error document (since most people’s reaction to that message will be, deservedly, “huh?”).
Actually, NewsBlur is named further down in the same article.
Further research led me to PEP 466 and Python’s issue 5639. It seems like Python 2.7.x does not have built-in SNI support yet, so that must explain the HTTP 403 mnot’s Apache configuration is sending.
(Edited to change the quote from the article to a better one.)
(Edited once more to add info about Python 2.7.x support of SNI.)
(Edited one more time to add a link to issue 5639.)