unable to startup sftp subsystem

unable to startup sftp subsystem

I am trying to do a ssh connection in php. But I get this error Unable to
startup sftp subsystem. I don't understand it. And I could not find much
in google. Please let me know how to debug this
$resConnection = ssh2_connect($strServer, $strServerPort);
if(ssh2_auth_password($resConnection, $strServerUsername,
$strServerPassword)){
//Initialize SFTP subsystem
$resSFTP = ssh2_sftp($resConnection); -->error coming here
}else{
echo "Unable to authenticate on server";
}