Run-Time Error 1004 on Certain Machines

We have a macro-enabled template which has been working fine on all machines in the office for ages. Recently, on certain machines only, it has produced "Run-time error 1004: Unable to get the Object property of the OLEObject class."

It works on one machine with Windows 7 Professional installed running the Home & Business 2013 version of Excel. It does not work on another Windows 7 Pro machine running the Professional Plus 2013 version of Excel.

The code which seems to be producing the error:

Option Explicit

Dim ComboBoxes() As New ITEM_COMBOBOXES

Sub InitializeBoxClass()

    Dim Obj As OLEObject
    Dim ComboBox_Count As Integer

    For Each Obj In PO_CREATION_WS.OLEObjects
        If TypeName(Obj.Object) = "ComboBox" Then  'DEBUGGER HIGHLIGHTS THIS LINE
            ComboBox_Count = ComboBox_Count + 1
            ReDim Preserve ComboBoxes(1 To ComboBox_Count)
            Set ComboBoxes(ComboBox_Count).ITEM_COMBOBOXES = Obj.Object
            Obj.ListFillRange = "ITEM_LIST"
        End If
   Next Obj
End Sub

I have tried referencing the OLEObjects by their index numbers instead of as a collection. This produces another error (not the same error as before). I have tried activating each OLEObject prior to retrieving its properties; another error code. I have tried adjusting the security settings in many different ways with no luck.

Does anyone have any idea what went wrong?


(Just in case, here is the ITEM_COMBOBOXES class)

Option Explicit

Public WithEvents ITEM_COMBOBOXES As MSForms.ComboBox

Private Sub ITEM_COMBOBOXES_KeyDown(ByVal KeyCode As MSForms.ReturnInteger, ByVal Shift As Integer)

With ActiveSheet
    Select Case KeyCode
    Case vbKeyTab
        If Shift = 1 Then 'SHIFT PRESSED, SHIFT TAB MOVES BACKWARD
            If Me.ITEM_COMBOBOXES.Index = 3 Then 'FIRST BOX SELECTED
                .OLEObjects(.OLEObjects.Count).Activate 'GO TO END
            Else 'NOT FIRST BOX, MOVE BACK ONE
                .OLEObjects(Me.ITEM_COMBOBOXES.Index - 1).Activate
            End If
        Else 'NO SHIFT KEY, TAB MOVES FORWARD
            If Me.ITEM_COMBOBOXES.Index = .OLEObjects.Count Then 'LAST BOX SELECTED
                .OLEObjects(3).Activate 'GO TO TOP
            Else 'NOT LAST BOX, MOVE FORWARD ONE
                .OLEObjects(Me.ITEM_COMBOBOXES.Index + 1).Activate
            End If
        End If
    Case vbKeyLeft
        Range("A" & Me.ITEM_COMBOBOXES.Index - 1).Activate
    Case vbKeyRight
        Range("C" & Me.ITEM_COMBOBOXES.Index - 1).Activate
    Case vbKeyDelete
        Me.ITEM_COMBOBOXES.Value = ""
    End Select
End With
End Sub