This is Ubuntu 23.10 running xfce4 on both systems. Totally wiped an older nxserver’s files by hand, as something was causing it to just show a very small window which was not the shared display. Reinstalled the server from the current deb here (free version, 8.11.3_4). Now getting the message in the subject line. Restarted the server, trying both Xorg and Wayland — same error.
This used to work fine between these systems, with the current OS versions and desktops.
Telling it to go ahead and create a new display, I get “Session negotiation failed.” It’s the shared display I’m after though. It’s a physical screen.