Hi! Newbie here, but I seem that usual reports are regarding Hookpad. I’m only have and been using the books, and just wanted to notice that if I open the lessons on book (didn’t test with the site) It blocks my midi ports to assign my midi controller (Komplete Kontrol) from any software I open in the back (kontakt, sine, ez-whatever).
Workaround is simple, just open the software player, assign midi port, then open hooktheory and then I can use it in the back. It’s way minor, but for new users that wan to open their own instrument to play along with the books, might get confusing the port can’t open 
. So maybe no need to fix, but to be aware of this situation if arises? I bet I’m the lucky single one.