Hello,
that is not coming from vbseo.
that's an IIS issue of folder protection or some other mod_xxxxx filter on your webserver for trying to LOAD the file. check that it has the proper 'chmod for windows' (whatever it is.. i'm not a server guy, especially windows) on the folder.
You should see a simple webpage login prompt, as per
http://www.vbseo.com/vbseo_sitemap/