The typical format of an url included in one of the rebelion.org rss feeds looks like this:
http://www.rebelion.org/noticias/ee....nicomio-148960
In order to avoid the 404 "Not found" problem, it is necessary to convert the URL
to the following format:
http://www.rebelion.org/noticia.php?...-al-manicomio-
Doing this for each of the url listed on the rss feed will solve your problem. However
I do not know how to do this using calibre/python regular expressions, although
one can presume that it must simple to do it, since it is rather straightforward
using set/awk.
Let us hope that one of the more knowledgeable users will help with this url
modification. Good luck!