I figured it out.
Posting here just in case someone has the same problem
While my site is getting updated i have a regular "under construction" on the site root index.html
I also have a index.php page that im going to use for my homepage when my site is done. The sitemap generates fine when i delete the index.html page.
I guess the software doesnt like it if there is a index.html and index.php page in the same directory.
Hope that helps someone else.