I know this is a very old thread but I came across the same issue and decided to share my solution here.
We get this error when using the RFC destination SAPFTP to an FTP server that allows connections only from the SAP Application server (as they might have firewall rules to prevent connections from any other systems). In that case you should use the RFC destination SAPFTPA instead.
If you don't find the RFC destination SAPFTPA in SM59 transaction, run the report RSFTP005 to generate it.