I have additional info following some test I made :

I added a property "Name" to clsObj01, and defined it in the formModule

Set obj01 = New clsObj01
obj01.Name = "Form Module Instance"

Later, just after I passed the object to the other class instances

Set obj0201.obj = obj01
Set obj0202.obj = obj01
Set obj0203.obj = obj01


I put this code :

Debug.Print obj0201.obj.Name (result: "Form Module Instance")
Debug.Print obj0202.obj.Name (result: "")
Debug.Print obj0203.obj.Name (result: "")

Now I can see why I get the error 91 when using the object in instances
02 and 03 of clsObj02, but I still no not understand why the actual
obj01 reference is not kept inside obj0202 and obj0203..

Still looking for ideas or explanation..