gdm X login does not work anymore after last update (dbus problem?)

[Stefan Bellon <bellon@axillion.de>]

I've done an apt-get update && apt-get upgrade on my notebook last
night and today I noticed that I can no longer login to an X session
using gdm.

I get the following information in my ~/.xsession-errors file:

/etc/gdm/PreSession/Default: Registering your session with wtmp and utmp
/etc/gdm/PreSession/Default: running: /usr/X11R6/bin/sessreg -a -w /var/log/wtmp -u /var/run/utmp -x "/var/lib/gdm/:0.Xservers" -h "" -l ":0" "sbellon"
/etc/gdm/Xsession: Beginning session setup...
Failed to start message bus: Failed to read directory "/etc/dbus-1/${prefix}/share/dbus-1/services": No such file or directory
EOF in dbus-launch reading address from bus daemon

I have not much clue what this error means apart from the fact that
there is no subdirectory hierarchy share/dbus-1/services
below /etc/dbus-1 and share/dbus-1/services is available under /usr.

Can somebody give me some clues about how to trace the problem down?

-- 
Stefan Bellon