Something’s wrong with our crawling, please help us figure it out

Greetings. We are having quite a few issues with crawling. First of all, we noticed that Rich Snippets aren't working; not only that, but the testing tool in Webmasters constantly says that "URL has not been found".
We've been trying with W3C validator and we got a 400 error; Xenu Link Sleuth returns a blank page as well. In Webmasters, it wouldn't seem like crawling is an issue, although there is a meaninglessly high number of 404s – almost a third of our submitted pages, but this is…
Something's wrong with our crawling, please help us figure it out

See original post by Alexious