make sure that you completed all steps during installation, including setting permissions for sitemap.xml file (0666 permissions) and generator/data/ folder (0777).
Thank you for your message.
However the sitemap was installed by yourselves about 2 years ago. As far as I'm aware permissions haven't changed. Ditto data.
I've looked through the configuration tab section and can't find anything that obviuosly relates to permissions.
I've also compared the config settings with another of your sitemaps that functions OK on another server for another of my sites. And can't see any differences.