I´ve finally been able to remove the error from the portal index without removing news, but those ATOM & RSS feeds do not appear anywhere yet.
I saw that lines 70 & 71 in index.php file are calling a variable in $lang_common.
So I edited ROOT/lang/English/common.php and:
// For extern.php RSS feed
'RSS description' => 'The most recent topics at %s.',
'RSS description topic' => 'The most recent posts in %s.',
'RSS reply' => 'Re: ', // The topic subject will be appended to this string (to signify a reply)
'ATOM Feed' => 'ATOM Feed',
'RSS Feed' => 'RSS Feed',
As I wrote, no error is displayed now but feeds are not shown either.