Root can't login (but can in single user mode)

[Xaveer Leijtens <xaveer@xlpc.et.tudelft.nl>]

I run debian woody on a number of machines, but on one of them I can't
login as root. Only if I reboot in single user mode, everything is fine.
I also tried to login via 'ssh root@localhost', and it does prompt for
(and accepts) the password, but stops after displaying /etc/motd. Same
happens on the console logins. Also 'sudo bash' prompts for my (user)
password, accepts it but doesn't continue. 

I've apt upgraded very recently (in single user mode), but this didn't
help. I also changed the root home directory to an empty directory, so
it can't be stuck in some of the bash rc files.
This has been going on for a few weeks now, and I've ran out of things
to try.

Any suggestion would be greatly appreciated.

Thanks,
Xaveer