I have 5.0.2. and when I exited the castle and got an egg from Rex, the game froze. I looked at the changelog for the version because I've glanced over it before and thought I remembered something mentioned about Rex and an egg. In my case my party was full, and the changelog mentioned making room in your party and getting the egg from below him. I'm not exactly sure how talking to him from below is even possible, since the event auto-activates instead of being static so you're not able to move below him. But anyway, my party is full and the castle doesn't have a PC, so I don't know if there's anything I can do until/unless demo 6 comes out with a fix for this.