Some times, you try to edit pages in SPD, you try to edit the HTML/script in some format that is not understandable to sharepoint'. Though it saves the script , some times throw the error. At this time application pool is currupted with the invalid formats. So, if you open the SPD again, server throws the error 'The server could not complete your request' .
Recycle the application pool.go to IIS -> web sites -> right click on the application pool -> recycle.