Comment 72 for bug 495361

I had the same problem with Xubuntu 16.04. It happened maybe a week after upgrading from Xubuntu 14.04 (I restart my machine everyday and always do the proposed updates). I could Ctrl-Alt-F1, login and startx to start a normal session. The problem appeared every time I logged in, and I have been using the workaround for maybe ten days before taking time to solve it better: it didn't disappear alone ;-)

Removing the .cache/sessions directory solved the problem. The old directory content is:

$ ls .cache.old/sessions
thumbs-kobolde:0 xfce4-session-kobolde:0.bak
xfwm4-203df6e15-7c7a-4316-aa41-1a78e9658f69.state
thumbs-kobolde:1 xfce4-session-kobolde:1
xfwm4-29d50ede6-120c-4261-9b1e-79331c3c42e6.state
xfce4-session-kobolde:0 xfce4-session-kobolde:1.bak

Hope this can help you to solve the problem.