So you've got a nicely configured server, slightly tarnished by the presence of Plesk but everything seems to be running well. Suddenly, you've got users complaining that they can't access the server via FTP.
You're running ProFTPD (as Plesk kindly installed it for you) and can log in from the CLI FTP client (on Windows or Linux), but can't get in using FileZilla, FireFTP or Internet Explorer. FileZilla is probably giving the error "Cannot Retrieve Directory Listing" but will have authenticated correctly just before that. For some, FileZilla will hang just after MSLD or LIST commands.
This documentation details how to resolve a common issue