Help -- Can't logon at GUI session (ctl-alt-F7)

[Dennis Wicks <wix@mgssub.com>]

I fill in user-id and password, click "Sign In" and the 
screen goes blank, flashes a few times, then comes right 
back to the sign in screen. Trying to log on with root does 
the same thing, even though I am supposed to be able to do it.

I can logon at terminal screens (ctl-alt-f1 etc) with either 
my personal userid or as root with no problem.

In case it makes a difference, Jessie AMD64

Many TIA!!
Dennis