If I disable the firewall (System>Administration>Firewall Configuration) then the sync works. The clue came from this forum
As recommended there, I checked security by visiting www.grc.com
and doing a ShieldsUP! test. Even with the firewall disabled, that test showed complete security for the computers on the LAN so should I bother with the firewall?
How could I discover the port(s) which need to be open for the sync to work? Then the firewall could be left enabled but with the specific port(s) left open.