This started happening since the last update for WP (3.6). I'm using Shortnotes (http://www.s5themes.com/theme/shortnotes/) and my webpage is edmw.com.ar.
When I go to "Posts" section, I see the following where it should be a description of the post:
"Warning: htmlspecialchars(): charset `UTF-7' not supported, assuming utf-8 in /home/juanjo/public_html/wp-includes/formatting.php on line 2751"
Also, when I go into posts I see that line on top of the browser, and down the title. And other codes over the place.
Finally the link to my last post (It was the only post I made after updating WP) is: http://edmw.com.ar/2013/08/warning-htmlspecialchars-charset-utf-7-not-supported-assuming-utf-8-in-homejuanjopublic_htmlwp-includesformatting-php-on-line-2751/.
Here you will see a screen in the "Edit Post" section of my last post: