An error that is still being looked into renders both, 32-bit and 64-bit installs unbootable.

This can afterwards only be fixed from either a live CD or a chroot.

We will keep you posted.

[Edit slam]: Here the background:
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and
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url was:"http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=581704"
linktext was:"http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=581704"
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Fixes:

A) Do not dist-upgrade yet

B) You dist-upgraded

You need to edit concurrency setting

sed -i"/^CONCURRENCY\=/d" /PATH_TO/etc/default/rcS

C) You already rebooted, and find your system hangs in the boot process

Boot again with cheat code 1 (gets you into runlevel 1), press Crtl+D when asked, and apply the fix B).

Your system will boot normally again after that.

Thanks to slh for the fix & hints!