DM crashes in Sid

[Mark Allums <maa@allums.com>]

Ihave Mate, Gnome Flashback, and XFCE installed.  Today, when starting 
up, all three will start, then exit with extreme prejudice.  Mate was 
doing that before, so, being lazy, I switched to XFCE, and didn't try to 
diagnose and solve and fix. But now all are doing it.  Also, the virtual 
terminals (Alt-Ctrl-F2, etc.) are behaving a bit weirdly, and will not 
show a bash prompt.

Oddly, when I rebooted and started in rescue mode, when I changed to 
user mode via SU, as in su george, I expected that typing startx, I 
might get some error messages.  But X works, and LightDM works, and when 
I logged into Mate, Mate continued to work, when it hadn't been working 
for months.

What do I need to do to discover the problem?  Running Sid.  I recall a 
similar problem for someone else, a segfault-or-other, I think, that I 
seem to recall involved deleting a config file.  If someone that recalls 
that could post a link, it'd be great, or could coach me how to diagnose 
and fix, or at least tell me what to Google for...

Thanks

Mark Allums