FEED RSS bug is almost making me lose my customer

Sorry my poor english, i’m from Brazil. But the fact is i’m really desperate, someone can help-me please? I really need to validate my rss feed to use in mailchimp rss campaings and another stuffs.
I already tried the use of the plugin rss fix error, but it did not work.
I’ve read about remove blank spaces in core files, i checked everything, but it did not work.
my costumer website is: sandrabastoscoach.com.br
I know that the validator accuses the problem of being in some specific posts, but I can not find anything wrong with these posts.