public static void Stop() {
		Bukkit.getScheduler().cancelAllTasks();
		CheckT.stopTimer();
		GameT.stopTimer();
		LobbyT.start();
		ArrayLists.Blue.clear();
		ArrayLists.BlueDead.clear();
		ArrayLists.Red.clear();
		ArrayLists.RedDead.clear();
		for (Player player : ArrayLists.Game) { // <---- Line 30
			ArrayLists.Game.remove(player);
			ArrayLists.Lobby.add(player);

			int X = (int) Main.main().getConfig().getDouble("Spawn.X");
			int Y = (int) Main.main().getConfig().getDouble("Spawn.Y");
			int Z = (int) Main.main().getConfig().getDouble("Spawn.Z");

			Location loc = new Location(Bukkit.getWorld("world"), X + 0.5, Y + 0.5, Z + 0.5);

			player.teleport(loc);
			player.getInventory().clear();
		}
	}

Line 82 in CheckT is the Stop() initialisation.

Plugin FrogBall v0.1 generated an exception while executing task 17
java.util.ConcurrentModificationException: null
        at java.util.ArrayList$Itr.checkForComodification(Unknown Source) ~[?:1.8.0_151]
        at java.util.ArrayList$Itr.next(Unknown Source) ~[?:1.8.0_151]
        at me.FrogTerra.FrogBall.Game.Stop(Game.java:30) ~[?:?]
        at me.FrogTerra.FrogBall.Timers.CheckT$1.run(CheckT.java:82) ~[?:?]
        at org.bukkit.craftbukkit.v1_12_R1.scheduler.CraftTask.run(CraftTask.java:58) ~[craftbukkit.jar:git-Bukkit-9060bfa]
        at org.bukkit.craftbukkit.v1_12_R1.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:52) [craftbukkit.jar:git-Bukkit-9060bfa]
        at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) [?:1.8.0_151]
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) [?:1.8.0_151]
        at java.lang.Thread.run(Unknown Source) [?:1.8.0_151]