issue with a 4xx client error

I'm getting a 4XX client error, I have never come across these before (maybe naively).

This is frustrating me, because my page seems to be working fine but when scanning it for SEO p开发者_如何学Gourposes I am getting two 4xx client error in

Put simply, what are they?

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The HTTP 1.1 RFC explains and lists all of the 4xx errors.

"The 4xx class of status code is intended for cases in which the client seems to have erred."