Looks like the access to web site from SharePoint Designer is denied.
To check it you can use two ways:
Go to Site actions menu, select "Site settings".
Then select "Site Collection Administration" and choose "Configure SharePoint Designer settings".
Click "Enable SharePoint Designer".
- SharePoint Central Administration level:
Go to SharePoint Central Administration Web site, select "Application Management", under "Web applications" select "Manage web applications".
Select your web application from list, from the "General Application Settings" page, select "Configure SharePoint Designer Settings".
Click "Enable SharePoint Designer".