SSH from one VM to anther keeps failing – under same NAT network

I’ve set up two vm running on same virtualbox host, they are working under same NAT Network and each has grained an IP. Their network setting looks like:

In order to make SSH works, I’ve followed instruction of this thread, and my port forward setting looks like:

The ssh_config and their network interface are default, default network interface looks like:

# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback

the ssh_config looks like:

Host *
#   ForwardAgent no
#   ForwardX11 no
#   ForwardX11Trusted yes
#   PasswordAuthentication yes
#   HostbasedAuthentication no
#   GSSAPIAuthentication no
#   GSSAPIDelegateCredentials no
#   GSSAPIKeyExchange no
#   GSSAPITrustDNS no
#   BatchMode no
#   CheckHostIP yes
#   AddressFamily any
#   ConnectTimeout 0
#   StrictHostKeyChecking ask
#   IdentityFile ~/.ssh/id_rsa
#   IdentityFile ~/.ssh/id_dsa
#   IdentityFile ~/.ssh/id_ecdsa
#   IdentityFile ~/.ssh/id_ed25519
#   Port 22
#   Protocol 2
#   Ciphers aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc
#   MACs hmac-md5,hmac-sha1,umac-64@openssh.com
#   EscapeChar ~
#   Tunnel no
#   TunnelDevice any:any
#   PermitLocalCommand no
#   VisualHostKey no
#   ProxyCommand ssh -q -W %h:%p gateway.example.com
#   RekeyLimit 1G 1h
    SendEnv LANG LC_*
    HashKnownHosts yes
    GSSAPIAuthentication yes

The ping command between these two vm works perfectly ok, but whenever I tried to ssh from PC-3 into PC-2, it keeps failing. I tried to uncomment the port 22 in ssh_config on PC-2, which produce the same fail result. My ssh with debug output looks like:

Can someone points out what mistake I’ve made in setting that causes the ssh failure? Thanks in advance.