reftex broken: "symbol's function definition is void: decf"

[almarex]

Hi,
I'm using emacs (21.4.1) and auctex (all from ubuntu edgy) as tex
editor, but somehow, for some tex files reftex is mysteriously broken.
Any reftex action (label, cite, table of contents) gives: "symbol's
function definition is void: decf" in the minibuffer. strange thing is
that it doesn't seem to have anything to do with the tex file, I've
copy pasted the exact same content of the file into a new file and
then it works and then when I change the name of that file to yet
something else it stops working again. Anyone have any idea what may
be going on here?

Thanks,
 Em