Dear Oleg,
Thank you for your reply.
It works now, after I have put the file in the mods folder.
I had put it in the wrong folder, one level higher.
But... still the permissions are set to 666, like sitemap.xml, ror.txt, urllist.txt
And the page cannot be opened in a browser.
How can the right permissions (755, I think?) automatically be assigned to sitemap.html?
Best regards,
Robert