Yesterday I was playing with a friend online. She was in my convoy when I started Horizon Tour from the place at the Horizon festival.
Starting the event would both give us the countdown to start.
4 out of 4 times, when the timer reached 0, so the moment the event would start, her Forza Horizon crashed to desktop.
However, we started Horizon Tour from the Festival Playlist twice without any problems.
We are both playing on (high end) PC, Windows 10, fully capable of running the game.