The fastmail.com links only work for 2 factor authenticated accounts, which are using specific application specific passwords from what I understand.
I set 2FA, set up specific app passwords and switched to imap.fastmail.com and everything
works.
Saw a post about only using username -- not
username@fastmail.com
This is INCORRECT -- Especially for people who use a different login domain other than fastmail.com
You must still use
username@your_chosen_fastmail_doman.tld
/cl