Further experimentation resulted in a workaround. I switched from GDM to LIGHTDM Configured lightdm the same as for Ubuntu 12.04 XDMCP works as documented. I can now login to the server from any thin client terminal.
Further experimentation resulted in a workaround.
I switched from GDM to LIGHTDM
Configured lightdm the same as for Ubuntu 12.04
XDMCP works as documented.
I can now login to the server from any thin client terminal.