Unable to type <alt-gr> symbols under X

[Nacho de los Ríos Tormo <irios@telefonica.net>]

I have just reinstalled Debian on a new hard disk (could've been
smoother) and have found this problem:

I use the Spanish locale. In the text console, the keyboard works fine,
and I can type all kinds of accented letters as well as those accessible
with the alt-gr key (@,#, the tilde, the euro symbol, the backslash,
square brackets, curly brackets). However in X, I get to the accented
characters fine (áéíóúñ), but not to the alt-gr characters.

This happens with KDE or Gnome, and with ISO8859-1 or ISO8859-15.

What can it be?

Thanks for your help,

Nacho de los Ríos