I've build a sitemap_index pointing to different sitemaps "sitemap1.xml" If they are uncompressed, they are successfully validated and accepted (Google Webmaster).
However, when compressed, they are not accepted.
I simply compressed them using gzip:
My server nginx is enabled to send compressed files.
This time I have not found useful the link in "Sitemap guidelines": Which could be a possible error and how to check for it?