====== SSH X11 forwarding ======
Check ssh server config:

<code>
# sshd -T | grep x11
x11displayoffset 10
x11maxdisplays 1000
x11forwarding yes
x11uselocalhost yes
</code>

Adapt /etc/ssh/sshd_config if different from above.

If it still doesn't work, disable IPv6 by setting AddressFamiliy to "inet":

<code>
##BAD:
# sshd -T | grep address
addressfamily any
listenaddress [::]:22
listenaddress 0.0.0.0:22

##Good:
# sshd -T | grep address
addressfamily inet
listenaddress 0.0.0.0:22
</code>

Restart sshd:

  systemctl restart sshd.service

====== VNC ======

If using the default Gnome Vino server, disable encryption which is incompatible with (some?) other clients:

  gsettings set org.gnome.Vino require-encryption false