I have seen that Webmaster tools is giving back a 404 error for the urls which are perfectly fine, like these ones:
Why can this happen? Should I just mark them as solved?
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When Google Webmaster tool return a 404 status code:
When you remove a page from your site, think about whether that content is moving somewhere else, or whether you no longer plan to have that type of content on your site.
So you shouldn't worry about it as your webpages are working fine. You just mark as fixed in Google Webmaster tool.