hi Ronald,
you probably have mydomain/comments/feed/ link somewhere on your WP pages or in your sitemap.xml thats why Google is trying to check it. However your WP does not generate the page for some reason (maybe you just dont have any comments to aggregate).
You can disallow Google bot (and others) to visit it by adding:
Code:
User-agent: *
Disallow: /comments/feed/
to your robots.txt (if theres none in the root dir of your WP installation just create one and give it chmod 755)
should help