Webmasters at Google Groups are reporting that some verified sites' sitemap URLs are not being accepted by Google Webmaster Tools. The error some webmasters are receiving is: "URL not allowed - This url is not allowed for a Sitemap at this location."
Susan Moskwa of the Google Webmaster Central team says that this is a problem that exists for those who have numerous domains in their accounts. The alternative way is to specify the Sitemap through robots.txt. She writes:
As you've noticed, our cross-submissions system may experience some problems if you have a large number of sites in your account. For now, I'd recommend submitting your Sitemaps via your robots.txt file, or submitting separate Sitemaps for each subdomain.
The robots.txt sitemap protocol is explained in this Google Webmaster Help document. Simply add the following line to your robots.txt file:
Sitemap: [path to sitemap]
Forum discussion continues at Google Groups.