ERR_SSL_PROTOCOL_ERROR: When the Handshake Fails Before the Certificate

The four "your connection is not private" certificate errors all happen after a successful TLS handshake. ERR_SSL_PROTOCOL_ERROR happens before one — the client and server never agreed on a protocol version or cipher, or the port is not speaking TLS at all. The causes in the order they bite, and how to read what your server actually negotiates.