Feed problem wordpress

I get this when I go to my http://www.SITENAME.com/feed
"This page contains the following errors:

error on line 1 at column 35: Unsupported encoding UTF-6
Below is a rendering of the page up to the first error."

What should I do ? I have tried to enter on the ftp / public_html/wp-includes and all the feed files had UTF-8....
and the sitemap xml have UTF-8...
and feedvalidator.org says
Sorry

This feed does not validate.

line 1, column 31: Unknown XML character encoding: UTF-6 [help]

<?xml version="1.0" encoding="UTF-6"?>

What should I do ?