W3C validator: "acknowledgment acknowledgment signal: unrecognized_name"

Question

I try to check my site on the W3C Nu Html Checker , but I get below the error, I don’t know why?

IO Error: confirmation message: unrecognized_name

So far, my site is working fine on SSL.

I tried the Confirmation Warning: unrecognized_name error , but without help.

any guide how to fix this is very helpful. I did not have that kind of server knowledge.