I think this is a long standing bug. When I use Cover View, I get large white boxes too. If you do this from Playing Now, you'll see the displayed cover art in the middle, just like in standard view. But everywhere else there was text, you'll see a large white box. If you do it from an Albums, or Artists, or Panes view, the whole screen is white. Moving the cursor to the left, you'll get the navigation pane. Up top, you'll get the player controls.
But none of this is useful. The Wiki says that Cover view is supposed to be the same as Standard View, but full screen without any "chrome"; I guess they mean window borders and things?
If you just want to see large album covers, use Display View. Though Display View does *not* show any metadata for the currently playing song.
This is one of the few remaining areas of MC for Mac that is still kind of "broken". I mean no insult. Just saying how it is.
Brian.