SharePoint Designer 2010 – The server could not complete your request

Today I had a user report that they were trying to use SharePoint Designer to edit the home page of their team site but it displayed an error message:

The server could not complete your request.
For more specific information, click the Details button.

Clicking the details button gave a lengthy response:

The clue is in this part of the message:

A Web Part or Web Form Control on this Web Part Page cannot be displayed or imported because it is registered as safe on this site.

As this is a web part page I was able to open the Web Part Page Maintenance by adding ?contents=1 to the end of the URL:

http://intranet/sitename/?contents=1

Now I can see the list of web parts, delete those listed as “ErrorWebPart” and edit the page in SharePoint Designer.