Face Time connects, but then immediately disconnects. How can I fix this?

[iMore Question]

Face Time connects, but then immediately disconnects

Since El Capitan installation, whether making or receiving a FaceTime call between 2 iMacs (one, late-2007, the other, late-2013) call appears to connect, but immediately disconnects. Any ideas? And are others experiencing this problem? Face Time previously worked on both machines.