This occurs across devices (this screenshot’s on a Chromebook, and I’m also seeing it on Firefox on Windows) as well as clients; Android app hangs on “Loading…”
I have the exact same problem, with the 500 error visible on desktop and “The server barfed!” on iOS, with WiFi either turned on or off. However, it worked perfectly just last night.
@orphum@etcngh I found the bug. Both accounts had a newsletter feed with an internal list ID that was making /reader/feeds crash while building the feed list. I fixed the parser so those feeds no longer cause a 500. This should be back to normal. Thanks for the detailed reports.