Scenario: When 'Threaded View' is made as only 'Default View' in Discussion Board, and clicked on the Bread Crumb gives error.
Details: 'Threaded View' technically requires additional query parameters (details of the item) to display the contents of the discussion board. Where as link on the Bread Crumb need 'All Items' mode kind of thing to display the items.
Hence if you try to remove any other view (like flat , subject) the link tries to load the Discussion Board in threaded view and since the extra parameters cannot be loaded, page ends up in error.
Fix: Create a new view 'Subject View' or 'XYZ View' and make it default. Now, the bread crumb view will pick up your new View instead of the Threaded View and everything works fine.
On a side note... you can have 2 default views for Discussion Board.