Connecting to a modern Xserver with an old client

[Michael K]

I'm trying to connect an old HP logic analyzer (circa 1998) to a modern Xorg server.
The error message I get is ...

Xlib: connection to "192.168.1.1:1.0" refused by server
Unable to open window on "192.168.1.1:1.0"

I have executed "xhost +" to allow access by any client and I am able to generate an xterm window from another modern computer on the network "xterm -display 192.168.1.1:1.0".
I have no way to know what the logic analyzer is actually trying to do or what it doesn't like.

On the X server side, it's started by gdm & I can't find out how to change the log level of Xorg (I think there is some gdm code that starts Xorg) ... 
/usr/libexec/Xorg vt2 -displayfd 3 -auth /run/user/1000/gdm/Xauthority -listen tcp -background none -noreset -keeptty -novtswitch -verbose 7 -core

What might be the issue with this old X client authenticating with a modern X server?