I got this error at a client this week: SPD2010 worked really good and I was creating workflows ed.
Then I changed the authentication provider from Windows to Claims based security (via powershell). Since then, SPD will no longer connect to my SharePoint.
IT gives me the following information-level error in the Microsoft Office Alerts event viewer:
Microsoft SharePoint Designer
An error occurred while trying to fetch data from your SharePoint site. Unexpected response from the server. The content type of the response is “”. The status code is “OK”.
The solution is that there can only be 1 binding on a web application. If you have more than 1, it will no longer work and you get this error.
What you need to do, is to extend your web application (central administration – application management – manage web applications)
If you open the site of that extented web app, SPD will connect to that site.