SharePoint Designer 2010 – The server could not complete your request. The content type of the response is

This is mostly happening due to the use of SPConfigModification.

Solution:
1. Open web.config
2. Search for section <system.serviceModel>
3. add the following tag to the section:
<serviceHostingEnvironment aspNetCompatibilityEnabled="true" />
4. done!

Original solution was found here