Validation problem: Target was not accessed

[archer]

I created a new sitemap.xml file using [ External links are visible to forum administrators only ]'s great tool at:
www.xml-sitemaps.com
but run into a problem when I try to validate it.
After using the form on your page at:
https://www.xml-sitemaps.com/validate-xml-sitemap.html
the results read as follows:
__________________________________________________________________
Schema validating with XSV 3.1-1 of 2007/12/11 16:20:05Schema validator crashed

The maintainers of XSV will be notified, you don't need to
send mail about this unless you have extra information to provide.
If there are Schema errors reported below, try correcting
them and re-running the [ External links are visible to forum administrators only ]: [ External links are visible to forum administrators only ]
   (Real name: [ External links are visible to forum administrators only ]
    Length: 25753 bytes

    Last Modified: Fri, 23 Sep 2011 19:37:24 GMT
    Server: Apache)The target was not assessed
__________________________________________________________________

A couple of other validators (including the one at:
[ External links are visible to forum administrators only ]
) tell me:
"This document was successfully checked as well-formed XML!"
so I would appreciate your help or advice about what might be the problem with
the validation on the [ External links are visible to forum administrators only ] form.
Thanks.

[XML-Sitemaps Support]

Hello,

sometimes W3C validator fails to validate correctly, you should submit sitemap to google webmaster account to check if it's parsed correctly:
[ External links are visible to logged in users only ]

[archer]

Thanks for your reply.   
Over the past few days, [ External links are visible to forum administrators only ]'s validation script (if I visited their form directly) DID validate the sitemap.xml file, but the script on your website gave the above error.  Then, I would only get the error from your script if I used "www." in front of the domain name.  (in other words, your script would validate the sitemap file if I omitted the "www".)  So, it seemed weird.  Anyway, thanks again.