Both RSS feeds are valid. They load locally, but connections from NewsBlur’s servers to OverBlog time out before any response comes back. The 505 shown in NewsBlur means a timeout.
Upsilandre’s latest post is January 2025, but its feed still works. Djayesse is actively publishing, with a post from October 11. This is a connectivity problem between NewsBlur and OverBlog. I’m not sure why it’s timing out but it works locally, so likely they are blocking NewsBlur.