FYI:
Yes, ffnet support is still broken.
The only fix I've found so far would involve packaging at least three more modules in the FFF plugin, one of which(cloudscraper) is Python3 only and would (theoretically) only work in Calibre 5+.
They (ffnet) are also still actively fixing issues with their migration, so I'm not rushing while it's still a moving target.
Further discussion in
github issue.