Workbooks.Open() fails in ThisWorkbook.Workbook_Open()

I would really appreciate some pointers as I have run into something where I clearly don't understand enough about Excel.

I have now reduced the problem to the following

Private Sub Workbook_Open()
Dim Xbook As Excel.Workbook
Dim Xsheet As Excel.Worksheet
Dim PlanName As String
Dim RC As Long

    Xbook = Application.Workbooks.Open(Filename:="G:\GENERAL\Databases\Depot Insights\conf\WarehousePvA" & "\" & "Plans.xls", ReadOnly:=True)
    Xsheet = Xbook.Worksheets("Plans")

    RC = 2
    With Xsheet
        While (.Cells(RC, 1).Text <> "")
            PlanName = .Cells(RC, 1).Text
        Wend
    End With
    
End Sub

Is this too early in Excel startup to start opening another Excel workbook?



---- orignal question ----
I have library code / classes that are successfully used in several Access based projects but one is generating an Error 91 object error when used in Excel.

The code creates a separate Excel instance (via a global variable gXapp - excuse the ugliness) then opens an input file. In Access this works fine, but in Excel I get an error when exiting the OpenXbook function that returns an Excel.Workbook.

If you step through the code, you can see the Excel instance in Windows Task Mgr, then the file contents in the Excel instance that is visible, then error as you exit the function.
I've even tried dropping the gXapp and altering OpenXbook to use Application.Workbook.Open and still the same problem.

Any ideas?

Gordon

ThisWorkbook contains

Private Sub Workbook_Open()
    InitialisePerformance
End Sub

A separate module contains

Global Const gcConfPath = "G:\GENERAL\Databases\Depot Insights\conf\WarehousePvA"

' Global variables for Performance
'
Global gXapp As Excel.Application

' InitialisePerformance
'
'
Public Sub InitialisePerformance()
Const cModuleName = "InitialisePerformance"
Dim Plans As ClassFileSystemFile
Dim Xbook As Excel.Workbook
Dim Xsheet As Excel.Worksheet
Dim PlanName As String
Dim RC As Long

    Set gXapp = CreateObject("Excel.Application")
    
    gXapp.Application.Visible = True
    
    Set Plans = New ClassFileSystemFile
        Plans.Path = gcConfPath
        Plans.Filename = "Plans.xls"
    
    Xbook = Plans.OpenXBook
    Xsheet = Xbook.Worksheets("Plans")

    RC = 2
    With Xsheet
        While (.Cells(RC, 1).Text <> "")
            PlanName = .Cells(RC, 1).Text
        Wend
    End With

Xit:
    If (Not (Xsheet Is Nothing)) Then Xsheet = Nothing
    If (Not (Xbook Is Nothing)) Then Xbook = Nothing
    If (Not (Plans Is Nothing)) Then Plans = Nothing
    Exit Sub
    
Err:
    MsgBox Err.Description, , cModuleName & "()"
    Resume Xit

End Sub

ClassFileSystemFile - includes

Private thisPath As String
Private thisFilename As String
Private thisFlagOpenReadOnly As Boolean
Private thisFlagCreateOnOpen As Boolean

Private Sub Class_Initialize()
    Set thisDictionary = New Scripting.Dictionary
    Set thisFSO = New Scripting.FileSystemObject
    thisFlagOpenReadOnly = False
    thisFlagCreateOnOpen = False
End Sub

Public Function OpenXBook() As Excel.Workbook
Dim Xbook As Excel.Workbook
Dim FullyQualifiedFilename As String

    If (Right(thisPath, 1) = "\") Then
        FullyQualifiedFilename = thisPath & thisFilename
    Else
        FullyQualifiedFilename = thisPath & "\" & thisFilename
    End If
    
    If (thisFlagCreateOnOpen) Then
        ' TBD - Factor out use of local var Xbook
        Set Xbook = gXapp.Application.Workbooks.Add
        Xbook.SaveAs Filename:=FullyQualifiedFilename
        Set OpenXBook = Xbook
        Set Xbook = Nothing
    Else
        Set OpenXBook = gXapp.Application.Workbooks.Open(Filename:=FullyQualifiedFilename, ReadOnly:=thisFlagOpenReadOnly)
    End If
    
End Function   ' ERROR OCCURS WHEN DEBUGGER STEPS OFF HERE