On delete in drawings, why "None of the Selected Items Could be Deleted"?

We have a dozen people running SW 2009 on a network. As of yesterday afternoon, when we attempt to delete a view, note, or other entity in a drawing, we get the message, "None of the selected items could be deleted."

How can I trace the cause of this message?

Thanks for your help.

SolidworksDrawings And Detailing