Web address in link causes errors (that are not of my making!)

I have a line of code in the Sitemap on my web site at
http://www.nigelbrown.me.uk/my-sitemap.htm that gives rise to 2 validation
errors and 3 warnings. It is a link to a website (the Guestbook). But these
"errors" and "warnings" are not problems with my code; it is because the
validation does not like the Guestbook web address.

How can I help this?

Thank you.

Nigel Brown