Hi,
I have a similar problem on my enviroment.
Cisco CUCM and IMP versione 10.5
Expressway C & E versione 8.5.1
When someone try to contat me via XMPP/Chat, I receive the message on my Jabber client but I'm not able to replay.
From log on IMP and Expressway I see that the XMPP PAcket is not forward from internal IMP Server to Expressway Core.
Cisco TAC engineering told me that it's know problem, to resolve it I need to stop XMPP federation on Expressway CORE and re-enable it and restart the IMP Server.
In my case this didn't clear the problem.
I'm still waiting TAC answer after last WebEx.