Why sshd won't talk on port 1723 but will on any other?

Ni!

I’ve correctly configured sshd to listen on port 1723, that is:

1. I have no other process listening on port 1723 (according to netstat)
2. Changing the number to any other available port works fine

Apparently, see logs below, it gets to the point that a connection (a tcp connection) is established, but doesn’t get through on the ssh level.

I need it to listen on port 1723 because, on a network I use often, it is the only allowed port besides 80 and the server already listens on 80 for web connections. I have never configured a VPN (port 1723 is usually a VPN port) on this server.

Any ideas? Any chance this is a problem from DigitalOcean’s side? Something within sshd?

This is what I get from the outputs/logs after the connection times out:

Client side:

$ ssh -vvv
Connection established.
...
ssh_exchange_identification: read: Connection reset by peer

Server side:

Log:
sshd[xxx]: Did not receive identification string from x.x.x.x

Netstat:
tcp        0     39 x.x.x.x:1723    y.y.y.y:44983    ESTABLISHED 23401/sshd: [accepted]

Thanks!