Missing XML tag This required tag is missing. Please add it and resubmit

Question

I'm having trouble with the following sitemap:

<?xml version="1.0" encoding="UTF-8"?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
  <sitemap>
    <loc>http://codingforspeed.com/forum/sitemap.php?fid=1</loc>
    <lastmod>2015-07-06 12:42:18</lastmod>
  </sitemap>
  <sitemap>
    <loc>http://codingforspeed.com/forum/sitemap.php?fid=3</loc>
    <lastmod>2015-07-06 12:42:18</lastmod>
  </sitemap>
  <sitemap>
    <loc>http://codingforspeed.com/forum/sitemap.php?fid=4</loc>
    <lastmod>2015-07-06 12:42:18</lastmod>
  </sitemap>
  <sitemap>
    <loc>http://codingforspeed.com/forum/sitemap.php?fid=5</loc>
    <lastmod>2015-07-06 12:42:18</lastmod>
  </sitemap>
  <sitemap>
    <loc>http://codingforspeed.com/forum/sitemap.php?fid=6</loc>
    <lastmod>2015-07-06 12:42:18</lastmod>
  </sitemap>
  <sitemap>
    <loc>http://codingforspeed.com/forum/sitemap.php?fid=7</loc>
    <lastmod>2015-07-06 12:42:18</lastmod>
  </sitemap>
</sitemapindex>

It is validated perfectly, but when I submit to google web master it complains:

Missing XML tag
This required tag is missing. Please add it and resubmit.

Solution

You're serving different sitemap content to Googlebot. This is what we (Google) get when we try to fetch your sitemap:

<?xml version="1.0" encoding="UTF-8"?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
</sitemapindex>

Try to fetch the sitemap itself with Fetch as Google in Search Console (former Webmaster Tools) or with a tool like Testuri)