If the domain is prompting for a login, and then gives “HTTP Error 401.1 – Unauthorized: Access is denied due to invalid credentials”, there might be an issue with the anonymous password that Plesk is holding. First thing to check is that all directory permissions are adequate and that anonymous directory access is ticked in IIS. If that fails try:
“C:\Program Files\SWsoft\Plesk\admin\bin\websrvmng.exe” –update-anon-password –domain-name=domainname.com
or for all sites:
“C:\Program Files\SWsoft\Plesk\admin\bin\websrvmng.exe” –update-anon-passwords-all