Comment 2 for bug 1768610

I have figured out the problem: the installer removed nux-tools, left over from unity, but did not purge it. This left a start up script in /etc/X11/Xsession.d which interfered with gnome. Purged that and the problem was gone.

So this is a dependency bug in the nux-tools package (or perhaps some mistakenly identified changed file which turned into evil cruft). Sigh. Took me a long time to find this.