Application using MIT-SHM extension create the shm segments with 0600
permissions, which means X server must be running either under the same user,
or under root to be able to access them.

Xvnc that is started by xinetd/systemd runs under the "vnc" user (not root for
safety and not a specific user, because Xvnc is started before any user is
selected in a graphical login). So every attempt to use MIT-SHM fails. Some
applications do not handle the failure gracefully (for example kdm).

MIT-SHM extension should be disabled for Xvnc.