Server Unexpectedly Closed Network Connection - Putty

Hi,

i received beaglebone black A6A yesterday.
192.168.7.2:3000 works well. But when i ssh to 192.168.7.2:22 using Putty on usb tethered and ethernet, “Server Unexpectedly Closed Network Connection” happened.
Then, i tried to scan opened port using nmap.

Starting Nmap 6.46 ( http://nmap.org ) at 2014-05-16 11:01 SE Asia Standard Time
Nmap scan report for 192.168.7.2
Host is up (0.0022s latency).
Not shown: 998 filtered ports
PORT STATE SERVICE
53/tcp closed domain
1723/tcp closed pptp
MAC Address: A2:F8:AD:69:BD:F5 (Unknown)

the 22th port is not opened yet.
so, whats wrong? is there any additional steps to connect via ssh?

Thank you,

Regards,

Faruq.