My sitemap.xml giving me - Your Sitemap appears to be an HTML page

Im using this plugins [INFINITE SEO AND W3 TOTAL CACHE]

I tried to fix my w3 total cache by excluding this: {followed as suggested}

sitemap_index.xml
post-sitemap.xml
page-sitemap.xml
category-sitemap.xml
post_tag-sitemap.xml
ngg_tag-sitemap.xml

First it solve the problem..

But today as i check it the errors went in again in webmaster tools. What went wrong? Pls help..

Thanks in advance.