My RSS Feed gives the error: XML declaration allowed only at the start of the document because of the string <!-- ngg_resource_manager_marker --> at the beginning of the file. I read that this is a common issue and if I try to disable nextgen the RSS Feed begins to work (also if I disable Yoast Wordpress SEO begins to work).
The feed is here: http://www.primapaginaonline.it/ppfeed
and the NextGen Gallery version is 2.0.66
Thanks