using err.raise works first time but subsequently produces and automation error

Hi All,
I have a generic function I use to validate if a value is within a drop down list. I recently decided to make it error within the function rather than passing it back to the calling proc, and I decided to do this by raising a custom error. This works as expected the first time it is invoked, but second and subsequent times produce an Automation error on the err.raise line. The complete sequence of events are:
1. user presses "Validate" button to start validation code
2. A user entered value fails the lookup and the expected error msgbox in the error handler is displayed
3. Usr clicks OK in msgbox and validation ends
4. User corrects the error (or not) and then again presses the validate button
5. Either the same value or another value fails the lookup function, but this time a "Run-time error '-2147220503 (800403e9)' Automation error is thrown
6. pressing debg takes me to the err.raise line
Full procedure shown below - Any advice welcome

Public Function DMLookup(LookupName As String, SearchValue As String, SearchCol As Integer, ResultCol As Integer, Optional IsCaseSensitive As Variant)
Dim LoopCounter As Long
Dim LookupFound As Boolean
Dim LookupToBeUsed As Variant
Dim result As String
    If IsMissing(IsCaseSensitive) Then
        IsCaseSensitive = False
    End If
    
    If gEnableErrorHandling Then On Error GoTo DMLookup_Error
        
    LookupFound = False
    LookupToBeUsed = Application.Names(LookupName).RefersToRange.Value
    For LoopCounter = 1 To UBound(LookupToBeUsed, SearchCol)
        'If its not a case sensitive check, wrap the check in UCASe to convert both values to upper case
        If IsCaseSensitive = False Then
            If UCase(LookupToBeUsed(LoopCounter, SearchCol)) = UCase(SearchValue) Then
                result = LookupToBeUsed(LoopCounter, ResultCol)
                LookupFound = True
                Exit For
            End If
        Else
            'If it is a case sensitive search, do a direct compare of the values
            If LookupToBeUsed(LoopCounter, SearchCol) = SearchValue Then
                result = LookupToBeUsed(LoopCounter, ResultCol)
                LookupFound = True
                Exit For
            End If
        End If
    Next LoopCounter
    If LookupFound = False Then
        Err.Raise (vbObjectError + 1001)
    End If
    DMLookup = result
DMLookup_Exit:
    On Error GoTo 0
    Exit Function

DMLookup_Error:
    If Application.ScreenUpdating = False Then Application.ScreenUpdating = True
    Select Case Err.Number
        Case vbObjectError + 1001
            MsgBox "The lookup for " & SearchValue & "withing the lookup name " & LookupName & _
            " has failed - This is often due to a copy and paste error where 1 lookup has been overwritten by another " & _
            " and so the validation stage passes initially, but is detected in the generation stage and so fails." & _
            vbCrLf & vbCrLf & "This is a critical error and any further processing will stop - Please correct the " & _
            "error before retrying", _
            vbCritical, "Error in function DMLookup of Module DeveloperToolKit"
        Case Else
            MsgBox "An unexpected error has occured, please contact Design with the below error details." & _
            vbCrLf & "Module = Module DeveloperToolKit" & _
            vbCrLf & "Procedure = DMLookup" & _
            vbCrLf & "Line = " & Erl & _
            vbCrLf & "Error Code = " & Str$(Err.Number) & _
            vbCrLf & "Error Text = " & Err.Description, vbCritical, "Error in in procedure DMLookup of Module DeveloperToolKit"
    End Select
    
    End 'Stop everything
End Function