Problem solved after #12 update ("Users could not use an FTP server as a remote backup repository if their FTP usernames contained the @ sign. (PPPM-1862)")
Same thing for me, also after today #6 update (12.0.18 update 6)
When I try to configure FTP Backup Server, I have error "Transport error: unable to list directory: Curl error: Login denied". Same server was working fine with 11.5