I had the same issue narcisgarcia. I wound up looking in the folder /usr/lib/cups/backend and found it empty. I also found a backend-available directory, with all the types listed, which seemed odd. So I linked things from backend-available into backend and the web GUI started working properly after that.

I figured it was because my setup was less than optimal, since I was hackishly installing things by hand with dpkg before it was all working with a simple apt-get cups But maybe that needs to be put into the setup script for cups, to make those links?