Join the game, you never spawn in or anything. Check the dev console client view and you get this stack trace. Starting happening today.
UPDATE: The error is unrelated to the broken game. The game broke because LocalScripts are no longer copied from StarterGui into PlayerGui. Writing a simple patch completely fixed the problem:
local contents = game.StarterGui:GetChildren() for i, v in next, contents do v.Parent = nil end game.Players.PlayerAdded:Connect(function(player) for i, v in next, contents do v:Clone().Parent = player:WaitForChild'PlayerGui' end end)