[Plugin: WordPress SEO by Yoast] Internal Servor Error 500 while editing Htaccess

I tried changing a redirect in the htaccess in the WordPress seo Yoast back office. When I changed my url, I had this 500 internal servor error.
I thought it was possible to edit htaccess directly in the wordpress seo Yoast interface. Next time I will do it using FTP. Any thoughts?

http://wordpress.org/extend/plugins/wordpress-seo/