x-window manager does not run after upgrade

[Matija Grabnar <matija.grabnar@arnes.si>]

I have a weird problem which came about after the latest upgrade of X
(I'm running debian unstable all the way). 

X starts correctly, I get the xdm dialogue, but when I log in with username
and password, I get the grey grid for something like 10-30 seconds, then
back to the xdm login dialog. 

However, the problem is not in xdm - if I kill xdm and log in on the console,
and issue startx by hand, I get the exact same response: grey grid, and
then back out. I can't see any error messages, just the usual informational 
messages.

Note that I had no problems before upgrade - fvwm started without problems
from xdm. 

Any ideas what I should look for to discover what the trouble is?