validator.w3.org validator error - Unable to show outline. What does this error mean?

Question

Google Chrome

Good afternoon.
When checking the site in the validator, inserting the URL, if you select the outline checkbox, it does not display the document structure and gives the error "Warning: Unable to show outline. (Most likely due to a malformed HTTP response from the server hosting the document that was checked.)"
If check the code for such an error, it gives out the structure.
Tell me what kind of error and how to deal with it?