I have used all types of FTP (FTP/FTPS/SFTP) successfully in the cloud so it is doable right now. But it does depend on the libraries you are using.
Based on the error message I am assuming you are currently trying to get ftp4j to work in your project? I have never used ftp4j myself. Since you are getting a security error it would require additional privileges. You can always enter a ticket and ask for the additional permissions and our team will evaluate if that could be enabled for all cloud nodes.
I have been using the FTP client from apache (org.apache.commons.net.ftp.FTPClient), which is part of the apache commons.net library. This works fine for me for both FTP and FTPS. For SFTP I use the jsch library (which is also used in the appstore module).
Here you can download an example of the code that i have been using
Does this problem occur locally? If yes, you can try to turn 'emulate cloud security' off.