Yes, the tickets go fast. You have to be literally waiting for the 10am open, or so I've heard from people who have been successful in getting them. I have tried several times and gotten the similar error about tickets being in other user's carts followed by a sold out. I'm also not sure how many tickets are set aside per show, but it seems like a fairly small amount given how well the show is selling. I'd suggest giving the box office a call and see what dates [if any] have 30 under 30 available and buy in advance since they are 32.
"Anybody that goes to the theater, I think we’re all misfits, so we ended up on stage or in the audience.” --- Patti LuPone.