Could be related to this:
502 Bad Gateway - FM Down? -
http://www.emaildiscussions.com/showthread.php?t=74009
- Bruce
edit: I am having problems connecting with FileZilla, but no problems (directory listing, changing, uploading) with the web UI. Fastmailstatus shows no issues with FTP, so maybe open a ticket if it's an immediate concern for you.
another edit: I just created a service ticket.