Try removing the fast passes for party day, finish booking all other days, then go back to the party day. And play around with the times a little. I could not book a fast pass starting at 330 without the error or one ending at 7. I ended up with 355 455 555. I know in the past they have gotten rid of anything that ended past 630 but they may have changed it up this year since I could not book starting at 330.
Anyway, I had the same error as you, but when I deleted party fast passes, made the rest, went back to the party and played with the times, I was able to do it.