When I tried to connect to macOS Mojave machine from other machine, I got the following message.
X11 forwarding request failed on channel 0And when using -v option, I got
debug1: Remote: No xauth program; cannot forward X11.
X11 forwarding request failed on channel 0So the problem seemed to be an issue of xauth-related, and actually this was because the remote server could not find the xauth program, even though I do not know the reason why..
I add a line XAuthLocation /opt/X11/bin/xauth to /etc/ssh/sshd_config on the remote server, to become as follows.
X11Forwarding yes
X11DisplayOffset 10
X11UseLocalhost no
XAuthLocation /opt/X11/bin/xauth
Problem solved.
References