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

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

I finally solved the problem. My /etc/environment had a single line
  LANG=nl_NL@euro
which (for unknown reasons) caused the problems. Removing this line did
the trick.

Now that I can login as root again, I still wonder why this is causing
trouble in the first place and also why it only prevents root from
logging in (and only in multi-user mode) and others still can login.

Anyway, thanks to all who replied and (indirectly) helped me to solve
the problem.
-- 
Xaveer Leijtens <Xaveer@xlpc.et.tudelft.nl>