[Plugin: Google XML Sitemaps] error in google webmaster google xml site map generator

google is not able to download my sitemap from the google site map generator suddenly. it has been working for about 10 months fine and today i started to get errors. i have resubmitted both the xml version and xml.gz versions to google webmaster and i get the following error from google webnmaster:

We encountered an error while trying to access your Sitemap. Please ensure your Sitemap follows our guidelines and can be accessed at the location you provided and then resubmit.

any ideas what is going on?

http://wordpress.org/extend/plugins/google-sitemap-generator/