Since the last update 1.28, every time Player 1 tries to enter the caves, the game will crash (CE-34878-0). If Player 2 tries to enter the caves, it will crash sometimes. If I load the game without a local second player, I can freely travel between main and cave areas without ever crashing.
Create new online game with Forest World Gen Size: Big and Cave World Gen Size either Big or Default. World Gen Branches: Never, World Gen Loops: Always for both Forest and Caves. Server is set to Local only, Endless, Players 2, Server Mode Online.