You did not change from FTP to SFTP, but from FTP to FTPS.
SFTP = SSH connection with FTP on top, based on port 22.
FTPS = FTP connection with SSL, based on port 21.
In order to use SFTP, make sure that port 22 is accessible and that the subscription has SSH privileges, at least chrooted SSH access. Only the subscription user, but no additional FTP user can access the storage space through SFTP, because only that one user has SSH privileges.