What informations give :

Print(ListZone.Count());
ListZone.Debug();

SCRIPT : string s = 'Array count: 1'
SCRIPT : string s = '[0] => Param2<vector,int><9800ccb0>'

.get() might bug, can you give a try to :

Print(ListZone[0]);

int i = 0;
Print(ListZone.Get(i));

By the way, how can you execute Enforce Script?

Surprisingly!

SCRIPT : Param2<vector,int><97c6c240>
SCRIPT : Param2<vector,int><97c6c240>

This part also now works correctly:

Print(ListZone.Get(0));

SCRIPT : Param2<vector,int><97c6c240>

However, this only led to clarification of the problem.

class CustomMission: MissionServer
{
	ref array<Param> ListZone = new array<Param>;

	override void OnInit()
	{

		Param2<vector,int> m_Zone;
		m_Zone = new Param2<vector, int>("3000 0 9000", 15);
		ListZone.Insert(m_Zone);

		ListZone.Debug();
		Print("===<1>===");
		Print(ListZone[0]);
		int i = 0;
		Print(ListZone.Get(i));
		Print("===<1>===");

		GetGame().GetCallQueue(CALL_CATEGORY_GAMEPLAY).CallLater(CheckZone, 10000, true);				

	}

	void CheckZone()
	{
		ListZone.Debug();
		Print("===<2>===");
		Print(ListZone[0]);
		int i = 0;
		Print(ListZone.Get(i));
		Print("===<2>===");
	}
}

In log:

SCRIPT : string s = 'Array count: 1'
SCRIPT : string s = '[0] => Param2<vector,int><97c6c240>'
SCRIPT : ===<1>===
SCRIPT : Param2<vector,int><97c6c240>
SCRIPT : Param2<vector,int><97c6c240>
SCRIPT : ===<1>===

SCRIPT : string s = 'Array count: 1'
SCRIPT : ===<2>===
SCRIPT : NULL
SCRIPT : NULL
SCRIPT : ===<2>===

Isn't that how I initialize a global variable?

The script is executed on the server in the mission file init.c.
The server version: 0.63.148743

I`m sorry! Found a bug!

The error here:
ref array<Param> ListZone = new array<Param>;

Correct option:

ref array<ref Param> ListZone = new array<ref Param>;