So I appear to be having similar problems to katenepveu and DandalftheWhite.
I'm using the latest version of FFF and have version 3.36 of Calibre installed. I've tried downloading the following stories using both my regular login and my secondary login. Neither of them work but I can access the stories through my browser and use AO3's download option. Actually there are at least 22 stories I can't download and all of them are locked/restricted stories.
This is the error message I get when I try downloading them with FFF.