Where does S2S stumble?

Question

XMPP

I decided to raise my XMPP with blackjack and no logs, paranoia tormented me. ( bad joke )
Tested the server on this service https://xmpp.net/
XMPP is implemented based on ejabberd
C2S connection went fine - Grade B ( B - due to diff-healman < 2048)
S2S gives an error " Error: Connection failed . "
SVR records:

_jabber._tcp.****a.im. 	SRV 0 0 5269 ****a.im
_xmpp-server._tcp.****a.im. 	SRV 0 0 5269 ****a.im
_xmpp-client._tcp.****a.im. 	SRV 0 0 5222 ****a.im

Part of the config:

###'  S2S GLOBAL OPTIONS
    s2s_use_starttls: true
    s2s_certfile: "/opt/ejabberd-16.03/conf/server.pem"

Certificate from Let's Encrypt
Part of the log:

2016-04-11 0*:*8:28.553 [info] <0.450.0>@ejabberd_listener:accept:333 (#Port<0.21181>) Accepted connection 208.68.163.218:51442 -> *.1*7.*.*8:5269
2016-04-11 0*:*8:28.992 [info] <0.2619.0>@ejabberd_s2s_out:wait_for_validation:418 Closing s2s connection: ****a.im -> jabber.org (invalid dialback key)

I created accounts for tests on different public servers, everything works ...
Tell me what to fix to pass the test?