Seems that I found the problem:
I have a main .htaccess file that redirects to 3 different folders. Each folder is for a different website (all in the same host).
One of the websites is in WP. The rewrite happens well.
BUT, when I try to generate a sitemap - the wp returns a 404 for the sitemap generator and thats why I get an error.
solution: I'll probably try to rewrite from the php and not from the htaccess.