Atlas doesn't handle Onionoo's HTTPS redirects correctly
After moving Onionoo to a host that provides HTTPS, or rather a host that enforces HTTPS by redirecting HTTP requests to HTTPS, Atlas has a problem: it doesn't handle HTTP 302 redirects correctly. It simply says "Backend error!" The problem goes away with HTTPS Everywhere enabled, because then the client rewrites the HTTP link to HTTPS. But without HTTPS Everywhere, the server sends a 302 and the client gets confused.
An easy fix is to replace the three HTTP URLs in Atlas' sources with HTTPS URLs. That's probably a good idea, regardless of the stated issue.