'Server could not complete your request' ; 'soap: Serverserver was unable to process request' while opening a page in SharePoint designer

 

The error that you encounter often shows up when we try to open the file in Read-only mode without checking it out. Such errors are usually associated with incorrect master-page associations. In order to sort this out either correct the errors in the associated master-page or check-out the file and then try to open it.

Remember this could be one of the reasons of such errors and provided above is only a workaround.