Hi
Now I'm not sure if this is the answer but could be related.
I have a parent protal with multiple child portals.
Now when I login as host to the Parent Portal and look at the chld portals it tell me the in Site Marketing that the site map is
http://www.parent.com/SiteMap.aspx?portalid=10 If I login to the child the the same entry shows as
http:///www.child10.com/SiteMap.aspx?potalid=10
Now for a number of these portals I have not knowingly generated a sitemap.xml, but they are there.
I wonder if you should use a 3rd party tool to generate a SiteMap.xml and FTP it to the SiteMap folder
Good luck
Dave
****** UPDATE ********
I FTPed the back end of my site and renamed the SiteMap.xml and then went to the SiteMap.aspx page, this regenerated the file.
Theory. Your SiteMap.xml is empty / corrupt - rename / delete it then try the SiteMap.aspx again