Como @Adrian mencionou, você precisa configurar o FileZilla para usar uma conexão criptografada ao falar com esse servidor FTP.
Citações do Wiki do FileZilla :
For a client to connect to a server using SSL, then the host for that connection needs to be set to FTPS. In FileZilla client this means prefixing the host with "FTPES://" for "explicit" FTPS, or "FTPS://" for the legacy "implicit" FTPS.
FTPS (SSL/TLS) is served up in two incompatible modes. If using explicit FTPS, the client connects to the normal FTP port and explicitly switches into secure (SSL/TLS) mode with "AUTH TLS", whereas implicit FTPS is an older style service that assumes SSL/TLS mode right from the start of the connection (and normally listens on TCP port 990, rather than 21). In a FileZilla client this means prefixing the host with "FTPES://" to connect an "explicit" FTPS server, or "FTPS://" for the legacy "implicit" server (for which you will likely also need to set the port to 990).
Dependendo de como o servidor foi configurado, uma dessas duas configurações deve funcionar:
-
ftpes://
na porta 21 -
ftps://
na porta 990