Ugh! I was up at 5:45a, refreshing the page until they went live. I really wanted the post-party, but I kept getting errors when I tried on 2 days. I was able to book a Seats and Sweets for my preferred day, so I took that and keep trying for the post-party. A time comes up for the post-party like there is availability, but when I try to book it, I get an error message. I finally tried calling a half hour ago. Took about 10 minutes to get an agent and suspected pretty quickly that I should hang up and call back for a different agent. She asked 3 different times what park I would be in, despite me asking for the Magic Kingdom Happily Ever After dessert parties. She stumbled around for about another 10 minutes, then put me on hold. After 10 minutes on hold, the call disconnected. So I'm now on hold for my second attempt at booking over the phone.
ETA: So second attempt netted me a more knowledgeable agent and some information but still no reservation for the post-party. She told me there are known errors occurring for both agents and guests, that there is still likely availability but it can't be accessed until the system is fixed, and to keep trying.