Validator question

 Dear W3C,

I have a question about the following warning message:
*Unable to show outline. (Most likely due to a malformed HTTP response from
the server hosting the document that was checked.) *

Example url is:
https://www.divehardtours.com/ panaszkezeles
https://www.divehardtours.com/a-nap-akcios-utazasa

If I check by url, always get this warning message,
but if paste the html source code in the textarea i get:

*Document checking completed. No errors or warnings to show. *

In the access.log
128.30.52.64 - - [28/Dec/2020:14:01:39 +0100] "GET /a-nap-akcios-utazasa
*HTTP/1.1*" 200 90204 "-" "Validator.nu/LV http://validator.w3.org/services"

Is is possible that the HTTP1.1 request is causing the issue?
Because we use HTTP2.

Thanks for your help,
Greetings
Zoltan (Dive Hard Tours Travel Agency)