A accidentally removed the Trash icon from dolphin, and created a new one, typing 'trash:' for the location. But when I clicked on it it would show the trash folder, but the 'Empty Trash' menu entry on the context menu was grayed out.
When I changed the location to 'trash:/', it started working correctly, and 'Empty Trash' was not grayed out anymore.
So Why is there a difference between 'trash:' and 'trash:/'?