Moved To A New Server

I recently moved one of my clients to a new server, but despite the move, I kept all info the same, including their FTP username/password for SimpleCMS to be able to access it.

It is a Plesk server, so files would be located under /httpdocs.

Well no matter what I supply, /httpdocs/, httpdocs/, httpdocs, /httpdocs/index.html, httpdocs/index.html
I keep getting an annoying popup message that says:

Oops! We looked for index.html in /httpdocs/ but that's not the page we recognise from the homepage you picked earlier.

Please try again

I even tried deleting the site completely, and re-creating it within the admin, and the result is the same, and it prevents my client from editing the website.

Can you let me know how to resolve this??

Thanks!