Facetime fails

I'm trying to connect via Facetime. The machines used are a Mac Mini running 10.9.5 and an iMac running Snow Leopard. Both have the latest updates for their respective systems.

A call from the Mini, results in the ringing tone on the iMac but after pressing 'connect' the message 'Facetime failed' appears. The same happens if a call is made from the iMac to the Mini. The firewalls on both machines are off and I can't see anything in either router that might impede a connection, unless Facetime requires UPnP, which I don't think it does. Since the call/ring element is getting through I don't see it as a firewall/router problem - it's the connection part that appears to be failing.

The only other point I can think of is that the AppleID of one caller is not a XXX@icloud.com email address

Has anyone any other ideas as I've run out of things to try. AFAIK it's not a requirement that both OSs must be the latest and greatest...