The error says:
This feed does not validate.
line 15, column 1: channel contains more than one generator [help]
If you look at the resulting output, it shows TWO lines with generator:
<generator>http://programming.has.no.com/2008/05/14/wordpress-rss-pages-plugin/</generator>
and:
<generator>http://wordpress.org/?v=3.3.1</generator>
Since WordPress SHOULD be the generator, it’s that plugin ‘WordPress RSS Pages Plugin’ that’s causing the issue. Remove it.