For some reason, Voicemail Notify messages coming from Asterisk Boxes - are not forwarded to the SIP Extensions by Kamailio.

This is the SIP Packet coming to the Kamailio:

2018/08/28 10:59:19.165644 1.1.1.1:5060 -> 2.2.2.2:5060
NOTIFY sip:11@3.3.3.3:21755;rinstance=e6369e13becab3ef;transport=tcp;alias=3.3.3.3~21755~2 SIP/2.0
Via: SIP/2.0/UDP 1.1.1.1:5060;branch=z9hG4bK5f628579;rport
Max-Forwards: 70
Route: <sip:2.2.2.2:5060;transport=tcp;ftag=0472e823;lr=on;nat=yes>
From: "asterisk" <sip:asterisk@1.1.1.1>;tag=as1833c329
To: <sip:11@3.3.3.3:21755;rinstance=e6369e13becab3ef;transport=tcp;alias=3.3.3.3~21755~2>;tag=0472e823
Contact: <sip:asterisk@1.1.1.1:5060>
Call-ID: 93183ZjE3YmI0OGYxMDNiMzhlNzVjZjRjNTc2NTMwZGNmYzk
CSeq: 103 NOTIFY
User-Agent: NUACOM
Event: message-summary
Content-Type: application/simple-message-summary
Subscription-State: active
Content-Length: 93

Messages-Waiting: no
Message-Account: sip:asterisk@1.1.1.1
Voice-Message: 0/0 (0/0)

And this is the message which I'm receiving in the Kamailio logs:
[RELAY] Error when forwarding Request
ERROR: sl [sl_funcs.c:362]: sl_reply_error(): stateless error reply used: I'm terribly sorry, server error occurred (1/SL)

This is the Relay route:

# Forwarding Statefully
  if (!t_relay()) {
xlogl("L_ERR", "[RELAY] Error when forwarding Request\n");
    sl_reply_error();
  }
  exit;