Using MacOSX to attach to RHEL7 via GDM

I'm upgrading some servers from RHEL 4 and 5 to RHEL 7, and am having problems connecting to GDM from my Mac.

On the Mac, I used to do

Xnest :screen-num -query hostname

and I'd get an X11 session to log in to.

MoboXterm on the PC works fine with RHEL7 - once I drilled the right holes in the firewall.

Anyone know what secret sauce is needed to get past the

Oh no! Something has gone wrong.
A problem has occurred and the system can't recover.
Please log out and try again.

-- Thanks, Verne