If the server is not on a private LAN or VPN, and can be accessed via the public internet, then we'd be happy to try connecting to it.
@Leo I have sent the server address as a private/direct message. Please let me know if I can help in any way.
I've spent a while trying to debug this this morning.
Opus doesn't seem to get any data back from the server at all, as far as I can tell.
I can't find anything to support the idea that a client should send a banner to the server. Other servers don't seem to expect this either.
I did find a post saying the banner from the server is sent encrypted, after encryption and key exchange has been negotiated. So one possibility is that the two sides cannot agree an encryption protocol to get that far. But you usually get a proper error message when that happens. Maybe other servers fall back on one of the disabled encryption protocols just to send a proper failure message (which doesn't really need strong encryption), but this server is configured so it won't do that (e.g. the other encryption methods are not only turned off, but completely removed from its code). That's just a guess, though, as I'm not an expert on SSH/SFTP at this low a level.
I'd expect things to work when we switch SFTP libraries in future, which is something we plan to do (for a few different reasons; better performance on top of newer algorithms). I can't say when that will happen, though.
Looking at Wireshark log, I see that the messages like banners aren't encrypted. The messages I mentioned in the log are in plain text. The log for Filezilla shows that Filezilla sent its banner first ("SSH-2.0-FileZilla_3.50.0"). After that, the server responded with its own banner ("SSH-2.0-ProFTPD 1.3.1. Authorized uses only. All...") immediately followed by the list of supported algorithms ("qcurve25519-sha256@libssh.org,ecdh-sha2-nistp256..."). I'm not familiar with SFTP protocol either, but it's just an observation from the monitored traffic.
Looking at SSH connection/handshake process, I see that this may be a standard step in SSH connection process.