non explained 555 error on webpage

Hi,

   I was working on this webpage.

https://simonlaub.net/Post/2024/Onwards2024/Onwards2024.html

  When I tried to validate it, I got

I got the following unexpected response when trying to retrieve <
https://simonlaub.net/Post/2024/Onwards2024/Onwards2024.html>:

555

But this error is not explained on?

https://validator.w3.org/docs/errors.html

So, what to do?
https://validator.w3.org/feedback.html

please help!

Thanks, in advance !

best wishes
-Simon