Fixed. steanne was right: the feed got switched to an RSSHub copy of Hacker News. RSSHub blocks us, so that copy was empty, and about 15,000 readers got moved onto it this morning.
A while back someone changed the address on their RSSHub copy to the real Hacker News URL, and NewsBlur wrongly marked the real feed as belonging to their copy. When RSSHub started failing today, NewsBlur tried to combine the two and kept the broken one.
Everyone is back on the real Hacker News feed now, with read stories and training carried over. I’m also shipping a fix so this can’t happen to other feeds.