Relationship <Relationshipname> was modified or deleted since the diagram was loaded. (Visual Database Tools)
A modified relationship is about to be saved. It is possible that a table involved in the relationship has been deleted from the database by another user.
Verify whether this relationship is meant to be modified. Changing or deleting a relationship for a table may affect other tables.
For more information about relationships, see Table Relationships (Visual Database Tools) and How to: Delete Relationships (Visual Database Tools).