I do not think it is backdoor. I would contact hosting provider for assistance, they have full access to the server.
Also i would try temporarily rename/remove files like: .htaccess php.ini and try to make sure there is no index.html/index.htm, also try to load site from within Incognito/Private browsing session (without cache).
Make sure that you uploaded files into proper directory that is assigned to the addon domain and addon domain goes to proper hosting (http://intodns.com)
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