I had the same problem in /wp-admin/ – it goes blank. My temporary solution is to use /wp-admin/edit.php instead. I works for me, and all functions in the admin area seems to work as it is supposed to.
I’m probably missing something essential in this discussion. Can’t you just assaign a post to both main- and sub category? If classical music reviews, for example, belong to both main category “Music” and sub category “Classical”…? And isn’t that the obvious way of categorizing posts…? But as I said, I’m probably missing something…
