Add the secure option from the Connection method of promise-ftp to the FTP Node settings.
The FTP does only support secure connections. This must be enabled in the ftp library by setting
secure: true in the Ftp.node.js where a new ftp is added.
FTPs, that only support secure connections can not be connected via the FTP Node otherwise (except if you edit the Node.js file manually, if you host n8n yourself).