Ensure that you haven't included a forward slash (/) in your site path. If this is the case, the server thinks it's a sub domain in the URL and the site won't load correctly.
To amend the site path follow the steps below:
Find the site under Direct Debits in eDD.
Click the dropdown arrow against Actions.
Click Change Information.
Update the Site Path to remove any forward slashes (you may use hyphens instead).
Click Next through the subsequent configuration pages until you reach the final page and press Save.
Now when clicking the preview this should load correctly.