Seems fanfiction.net is somehow blocking FanFicFare resulting in a 403 for some stories like this one:
https://www.fanfiction.net/s/13422705/1/Troubled-Waters
It's not user agent based, adding one to personal.ini changed nothing.
I use Calibre 5.7.2 and FanFicFare v3.26.0 on Kubuntu 20.10.