Thank you for your response.
Unfortunately, it's not just one "sitemap.html" file. The xml-sitemap created thousands of files and folders with owner uuuuuuuu and permissions 644 under the "data" and "data/progress" folders. There's a corresponding fopen, fwrite and fclose error message for every one of these thousands of files.
Xml-sitemap didn't have problem creating these files, why didn't xml-sitemap create these files with correct permission in the first place?
It there an automatic way to handle this issue?