Error Code when attempting to generate sitemap

[karmal]

My website works fine when accessed through a browser, but when attempting to generate a sitemap, the following error comes back-

An error occured
There was an error while accessing the URL specified: <b>[ External links are visible to forum administrators only ]</b>
Please make sure to specify the correct website URL and resubmit your request.

I have not had a problem generating sitemaps from this website prior to last Friday.

I have noticed other users have had this same question and the reply has always been the server is coming back with a 404 code, however I don't see where it is explained how to fix this.

I have contacted my host and they have no idea what I'm talking about.  Can someone please provide a little more information as to how this can be resolved and why it happened in the first place?

Thanks!

[xml-sitemaps]

Hi, your site is not returning anything at all from our server. It may be a temporary issue or your hosts could be blocking access to our ip address.