This is strange. I got through almost all of the sites on the very frist instance.
Maybe you should regenrate your Sitemap again in the XML format and try submitting this to google. If it gets accepted, it means the sitemap is all okay. Then retry with Bing and if it fails, maybe you should contact them.
Also, please ensure if your site is verified at Bing and you have their codes on your website.
You can submit your sitemap from within the Dashboard for your site in Webmaster Tools. Look for the Sitemaps widget and click Submit a Sitemap to show a text box in which you can enter the location of your sitemap file.