Hi there,
I’ve added a contact to Jami - the invitation was acceted ok but Jami is stuck on “Downloading Conversation from Peer”. I have forced stop Jami but relaunching resules the message, preventing me to send or receive messages.
May have resolved it, I initiated a voice call between the two devices. This cleared the sync message. tested all ok
I’d suggest this is likely a synchronisation thing, and forcing the connection via a call forced the synch. Just a thought! 
That only seems to have come up once, forcing the client to deal with an incoming voice/video call forced it to resolve.