putty - server unexpectedly closed network connection on windows

I'm getting this error when using Putty:

server unexpectedly closed network connection

I try to connect my EC2 ubuntu 14.04 LTS Instance using putty, and also i generate pem key to ppk key then i upload ppk key in putty and try to connect my Instance.

Solution

A PuTTY session left idle will disconnect at a time determined by the host server. You can enable keep-alives in PuTTY to prevent the session from timing out.

To create and save a new keep-alive connection, follow these steps:

Open the PuTTY application, and go to the Options panel (labeled "Category") on the left of the window.
Select (click) the "Connection" item.
In the "Sending of null packets to keep the session active" area on the right, change the default value of "Seconds between keepalives" from 0 (turn off) to 1800 (30 minutes).
Select the "Enable TCP keepalives (SO_KEEPALIVE option)" check box. Note: This option may not be available in older versions of the PuTTY client.
On the topmost left side of the Options panel, select (click) "Session".
In the "Host Name (or IP Address)" field, enter the destination host name or IP address (e.g., "destination.ipaddress.here.com" or "192.168.1.1").
In the "Saved Sessions" text-entry box, provide a name for the session (e.g., "savedsession").
Select "Save".

To use the modified session settings, select it from the "Saved sessions" list, then click the buttons marked "Load" and "Open".

If your connected sessions still time out, enter a lower number of seconds into the "Seconds between keepalives" value.

On searching more I also got a point that:

"Uncheck 'Attempt GSSAPI authentication (SSH-2 only)' in Putty: Category - Connection - SSH - Auth - GSSAPI

In my case it seems to be GSSAPI is incompatible to a Ubuntu host that uses Beyond trust (formerly: likewise open)."