Not sure weather it's a GDM OR XDM issue, judging from other issues
found it looks like it's a previous GDM bug resurfaced!
** Package changed: xdm (Ubuntu) = gdm (Ubuntu)
--
You received this bug notification because you are a member of Ubuntu
Desktop Bugs, which is subscribed to gdm in
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.
** Changed in: gdm (Ubuntu)
Assignee: (unassigned) = James Anderson (jim