I’m a couple of lectures later now but I also found similar behaviour in the finished project on the course page: pressing the host-button with multiple editor instances running the game would crash it.
I think the delegates “act as one” when using Play In Editor, which causes the crashes. I initially had an instant crash when trying to Host with multiple instances of the game running in the editor. Recompiling the main menu level blueprint fixed this crash, but now, when I would press Host, all the players will WorldTravel to host a game.
Moving functionality back to the function that handles “Host” keypress instead of OnCreateSessionComplete returned the PIE functionality back to where it was, and it can still be played as a standalone.