Anthony Scolaro on "[Plugin: WordPress SEO by Yoast] error on line 1 at column 6: XML declaration allowed only at the start of the do"

I'm receiving this error on our production server but not on our development machines. Could this be a server issue? Both servers are ubuntu 12.04 with a regular LAMP stack...

error on line 1 at column 6: XML declaration allowed only at the start of the document

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