Some WordPress functions outputs character entity references. The most of these are not allowed in XHTML. So if your WordPress installation uses the mime type ‘application/xhtml+xml’, the generated sites could have invalid xhtml code and the browser would display the syntax error page.
Convertentity converts all entity references into their corresponding characters with the exception of the references for the following characters: ‘<>&»
Theses references are the only ones which are allowed in XHTML.
Te puedes descargar este plugin para utilizarlo en tu instalación autoalojada de WordPress.