SOLVED!! - does console HAVE to be installed on gateway Hyman Kaye 19.Jul.15 01:40 AM a Web browser Sametime Gateway 9.0Windows
To clarify Ben's comment - the path documented is not visible. The actual path is Application Servers > RTCGWServer > Java and Process Management (that's the missing bit both in the documentation and in Ben's comment)> Process Definition > Java Virtual Machine > Custom Properties > Add new property. In fact, it worked even when I only added the new property in Administration > Custom Properties but for safety I have it in both.
Further clarification is that the problem comes if you have both IPv4 and IPv6 active, but you are not defining IPv6 full addresses. In my case this was a test lab, and no DNS was set up, so all addresses were defined in hosts files, using only IPv4.