You are probably well aware that the achievements dont work, but I get an error message right after defeating the monster:

NullReferenceException: Object reference not set to an instance of an object
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AILayer_Attitude.FortressSwap (.Fortress fortress, Int32 oldOwnerEmpireIndex, Int32 newOwnerEmpireIndex)
AILayer_Attitude.OnFortressEventRaised (Amplitude.Unity.Event.Event raisedEvent)
AILayer_Attitude.EventService_EventRaise (System.Object sender, Amplitude.Unity.Event.EventRaiseEventArgs e)
Amplitude.Unity.Event.EventManager.Notify (Amplitude.Unity.Event.Event notification)
DepartmentOfTheInterior.SwapFortressOccupant (.Fortress fortress, .Empire newOccupyingEmpire, Boolean hasBeenExchanged, System.Object[] parameters)
DepartmentOfTheInterior.SwapFortressOccupant (.Fortress fortress, .Empire newOccupyingEmpire, System.Object[] parameters)
DepartmentOfTheInterior+<SwapFortressOccupantProcessor>c__Iterator22E.MoveNext ()
Amplitude.Coroutine.Run ()
GameClient.ProcessOrders ()
GameInterface.UpdateMessageBoxAndProcessOrders ()
GameClient.UpdateMessageBoxAndProcessOrders ()
Session.Update ()
Amplitude.Unity.Session.SessionManager.LateUpdate ()

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V1.5.1 S3  (64-bit)

Saveggames are attached, before and after the battle

Vaulters - all fortresses+monster.zip

Vaulters - all fortresses+monster1.zip