martin wrote:
    Check the server's log to see why it closes the connection.    
I don't see anything relevant, that is a connection attempt from the server side with debug3 enabled:
Mar  9 11:46:35 sms sshd[12426]: Connection from ::ffff:194.106.17.242 port 56505
Mar  9 11:46:35 sms sshd[10777]: debug1: Forked child 12426.
Mar  9 11:46:35 sms sshd[12426]: debug1: Client protocol version 2.0; client software version WinSCP_release_3.8.2
Mar  9 11:46:35 sms sshd[12426]: debug1: no match: WinSCP_release_3.8.2
Mar  9 11:46:35 sms sshd[12426]: debug1: Enabling compatibility mode for protocol 2.0
Mar  9 11:46:35 sms sshd[12426]: debug1: Local version string SSH-1.99-OpenSSH_3.8.1p1 Debian-8.sarge.6
Mar  9 11:46:35 sms sshd[12426]: debug2: Network child is on pid 12427
Mar  9 11:46:35 sms sshd[12426]: debug3: preauth child monitor started
Mar  9 11:46:35 sms sshd[12426]: debug3: mm_request_receive entering
Mar  9 11:46:35 sms sshd[12426]: debug3: monitor_read: checking request 0
Mar  9 11:46:35 sms sshd[12426]: debug3: mm_answer_moduli: got parameters: 1024 2048 8192
Mar  9 11:46:35 sms sshd[12426]: debug3: mm_request_send entering: type 1
Mar  9 11:46:35 sms sshd[12426]: debug2: monitor_read: 0 used once, disabling now
Mar  9 11:46:35 sms sshd[12426]: debug3: mm_request_receive entering
Mar  9 11:46:35 sms sshd[12426]: debug1: do_cleanup
Mar  9 11:46:57 sms sshd[10777]: Generating new 768 bit RSA key.
Mar  9 11:46:57 sms sshd[10777]: RSA key generation complete.