Cannot reproduce after disabling IPv6 and rebooting.
I am experiencing this issue on Prosody 0.9.10, out of the blue. It was working before. I disabled IPv6 system wide and rebooted and it seemed to fix it for now. It failed for every s2s query. When running ping on the hostname it worked fine and returned the correct IP.
I think it may be related to https://groups.google.com/forum/#!topic/prosody-users/45tMbXEqD3U
Oct 11 16:50:17 adns debug Sending DNS query to 192.168.1.1
Oct 11 16:50:17 adns debug Reply for pubsub.chatsecure.org. (thread: 0x1f21800)
Oct 11 16:50:17 mod_s2s debug DNS lookup failed to get a response for pubsub.chatsecure.org.
Oct 11 16:50:17 s2sout1f1ded8 info Out of connection options, can't connect to pubsub.chatsecure.org
Oct 11 16:50:17 mod_s2s debug No other records to try for pubsub.chatsecure.org - destroying
Thanks for the report, sorry about the slow response time.
As mentioned in that thread, getting a packet capture of the DNS request and response (if any) would be needed to diagnose this.