This is because your SiteBuilder index page is .htm and the Default Plesk index page is .html. You will need to go in and manually delete the index.html page in your file manager. Follow the dierctions below.
1) Login to your account/control panel
2) Click on the SYSTEM tab
3) Click on the domain you are working with
4) Click on the FILE MANAGER icon in the HOSTING section
5) Scroll down to find the HTTPDOCS folder (NOT the HTTPSDOCS folder)
6) Scroll down to find the INDEX.HTML file (DO NOT CLICK ON IT)
7) Check the box all the way to the right of that files row. This will select the file.
8) Once the box is checked, scroll back to the top of the page to find the REMOVE SELECTED link at the top right. It has a big red "X" next to it. Click it.
9) To confirm removal, check the CONFIRM REMOVAL box and click OK