Another protocol on that port

  ✗ No SFTP server at ssh.example.com:21: this port speaks FTP.
    It answered with an FTP greeting (220), not an SSH one.
    SFTP is usually port 22: remove :21 from server (sealup.yml line 4).

    sealup did not log in. Nothing was changed on production.
    More: sealup.dev/e/wrong-protocol

Exit code: 5. Server changed: no.

What happened

sealup reached the server, but the port in server: greeted it as an FTP server does (a line starting with 220), not with SSH’s greeting. SFTP and ssh:// run over SSH. sealup stopped right there: no user name or password was sent.

Why

The port in the address is the FTP one, most often 21. Or the host offers only FTP at that address.

The fix

Use the SSH port your host gives: 22 by default, so remove the port from the address, or write the one your host names:

server: sftp://deploy@ssh.example.com/home/site/www

If the host offers only FTP, write ftp:// instead. Plain FTP sends your password and files unencrypted: sealup says so on every run.