See
post #4997.
tl;dr for the last few pages and the GitHub thread: FFnet was being attacked by bots and installed Cloudflare. Cloudflare thinks FFF is a bot attacking the site and blocks it. Jim made a test version of FFF that doesn't get blocked.
The test version only really works on 5.x. Older versions just produced a lot of OpenSSL errors whenever downloading from FFnet.
If you're using Calibre 4.23 and don't want to upgrade for whatever reason, my suggestion is to
grab the portable version of 5, load up the test version of FFF in it, open your library, update, and then switch back to main Calibre for your reading pleasure.
Do not share libraries if you're using any version below 4.23 or
things will break.