tunneling an ssh session with russh

[StPanning]

Hi all,
I have trouble tunneling an ssh session with russh (local port forwarding).
I've written a small server (my-server) and tunneling clear text via nc/ nc -l works fine.

When tunneling a ssh session however, I run into problems.
the setup is
openssh-client -> my-server -> target machine/open-ssh server

the ssh command is:
ssh my-server -p xxxxx -L6000:target-machine:22
then
ssh my-server -p 6000

the debug-output I receive from the on the client side when connecting to the target-machine through the tunnel:

[...]
debug1: kex: algorithm: curve25519-sha256
debug1: kex: host key algorithm: ssh-ed25519
debug1: kex: server->client cipher: chacha20-poly1305@openssh.com MAC: <implicit> compression: zlib@openssh.com
debug1: kex: client->server cipher: chacha20-poly1305@openssh.com MAC: <implicit> compression: zlib@openssh.com
debug3: send packet: type 30
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
Bad packet length 1198486560.

Wireshark doesn't recognize the kex-reply from 'my-server' as ssh-protocol.

the code is pretty straight-forward and I don't know if I'm doing something wrong or if it is a russh-bug (maybe some bytes stripped or added)

async fn copy_to_source(source_stream: Arc<Mutex<ChannelStream<Msg>>>, target: Arc<Mutex<TcpStream>>)->Result<(), TunnelError>{
    let mut buffer = [0u8;4096];
    loop {
        debug!("tunnel_handler: copy to source");
        let mut target_guard = target.lock().await;
        let mut source_guard = source_stream.lock().await;
        let bytes_read = target_guard.read(&mut buffer[..]).await.map_err(|e| TunnelError { kind: IoError(e.to_string()) })?;
        if bytes_read == 0{
            break;
        }
        debug!("tunnel_handler: writing all: bytes_read {bytes_read} {:#04X?}", &buffer[..bytes_read]);
        source_guard.write_all(&buffer[..bytes_read]).await.map_err(|e| TunnelError{kind: IoError(e.to_string())})?;
    }
    Ok(())
}

copy_target is written the same, with source and target flipped

the-target machine is a local VM, no router, switch or firewall is between source and target.
A direct connection to the open-ssh server, without my-server in between, works.

has anyone an idea, what the the problem is?

TIA