Validate RSS gives 404

I’ve created two different blogs on different servers. One of them validated yesterday using https://www.feedvalidator.org/ the other consistently gave — Server returned HTTP Error 404: Not Found

Today – both of them give 404

I have not modified the wp-rss2.php

If I try to validate the directory name – leaving out the .php part, https://www.feedvalidator.org/ does not get a 404 error, but of course it does not validate.

If I paste the URL into a browser – it works and displays the XML