Now I am even more puzzled. I have been working almost non-stop all day on this, and have 6 of the 9 sites with sitemap.xml. Trouble is that I am sure I did not start the generator on at least a couple of those.
Anyway; I have spent a lot of time trying to detect minute differences which might make trouble, but haven't seen any. All sites are on the same host server, too.
It is near midnight over here in Australia! These used to all be frontPage sites, and on one I even removed it entirely through file manager on the ControlPanel, before completelely uploading it again, and the generator still would not work. Of course I removed all frontPage related stuff. Or did I? I am off to check that too now.