As a quick sanity check, I tried to open an xterm on adaql3 with DISPLAY=hapc7:0. Result:
xterm Xt error: Can't open display: hapc7:0
However, with DISPLAY=adaql1:0, opening an xterm works just fine.
Verified that xhost + was done hapc7. Still no luck.
Conclusion: something on hapc7 is misconfigured.
Found X server running with "-nolisten tcp" option. Security default. In /etc/X11/gdm/gdm.conf, added a line DisallowTCP=false. Restarted gdm. X no longer running with -nolisten, as expected. Gave xhost +. Tested with xterm. Works.
While I am typing, the Monticello display appears. All is good.
A copy of this log entry has been emailed to: rom@jlab.org