Такая проблема может быть вызвана тем, что Excel в момент экспорта может быть запущен, перед экспортом надо проверять открыт ли Excel. Причём, если ты работаешь по Win2000/XP, при просмотре в Task Manager->Applications Excela в списке запущенных приложений может и не быть, он может быть в списле процессов, его надо оттуда удалить.

В ручную это конечно утомительно каждый раз проверять, вот тебе пример кода:

Private Const SW_HIDE = 0
Private Const SW_SHOWNORMAL = 1
Private Const SW_NORMAL = 1
Private Const SW_SHOWMINIMIZED = 2
Private Const SW_SHOWMAXIMIZED = 3
Private Const SW_MAXIMIZE = 3
Private Const SW_SHOWNOACTIVATE = 4
Private Const SW_SHOW = 5
Private Const SW_MINIMIZE = 6
Private Const SW_SHOWMINNOACTIVE = 7
Private Const SW_SHOWNA = 8
Private Const SW_RESTORE = 9
Private Const SW_SHOWDEFAULT = 10
Private Const SW_MAX = 10
Private Const WM_CLOSE = &H10
Private Const INFINITE = &HFFFFFFFF

Private Declare Function apiFindWindow Lib "user32" Alias _
"FindWindowA" (ByVal strClass As String, _
ByVal lpWindow As String) As Long

Private Declare Function apiSendMessage Lib "user32" Alias _
"SendMessageA" (ByVal hwnd As Long, ByVal Msg As Long, ByVal _
wParam As Long, lParam As Long) As Long

Private Declare Function apiSetForegroundWindow Lib "user32" Alias _
"SetForegroundWindow" (ByVal hwnd As Long) As Long

Private Declare Function apiShowWindow Lib "user32" Alias _
"ShowWindow" (ByVal hwnd As Long, ByVal nCmdShow As Long) As Long

Private Declare Function apiIsIconic Lib "user32" Alias _
"IsIconic" (ByVal hwnd As Long) As Long

Private Declare Function apiPostMessage _
Lib "user32" Alias "PostMessageA" _
(ByVal hwnd As Long, _
ByVal wMsg As Long, _
ByVal wParam As Long, _
lParam As Any) _
As Long

Private Declare Function apiWaitForSingleObject _
Lib "kernel32" Alias "WaitForSingleObject" _
(ByVal hHandle As Long, _
ByVal dwMilliseconds As Long) _
As Long

Private Declare Function apiIsWindow _
Lib "user32" Alias "IsWindow" _
(ByVal hwnd As Long) _
As Long

Private Declare Function apiGetWindowThreadProcessId _
Lib "user32" Alias "GetWindowThreadProcessId" _
(ByVal hwnd As Long, _
lpdwProcessID As Long) _
As Long

Function fCloseApp(lpClassName As String) As Boolean
'Usage Examples:
' To close Calculator:
' ?fCloseApp("SciCalc")
'
Dim lngRet As Long, hwnd As Long, pID As Long

hwnd = apiFindWindow(lpClassName, vbNullString)
If (hwnd) Then
lngRet = apiPostMessage(hwnd, WM_CLOSE, 0, ByVal 0&)
Call apiGetWindowThreadProcessId(hwnd, pID)
Call apiWaitForSingleObject(pID, INFINITE)
fCloseApp = Not (apiIsWindow(hwnd) = 0)
End If
End Function


Function fIsAppRunning(ByVal strAppName As String, _
Optional fActivate As Boolean) As Boolean
Dim lngH As Long, strClassName As String
Dim lngX As Long, lngTmp As Long
Const WM_USER = 1024
On Local Error GoTo fIsAppRunning_Err
fIsAppRunning = False
Select Case LCase$(strAppName)
Case "excel": strClassName = "XLMain"
Case "word": strClassName = "OpusApp"
Case "access": strClassName = "OMain"
Case "powerpoint95": strClassName = "PP7FrameClass"
Case "powerpoint97": strClassName = "PP97FrameClass"
Case "notepad": strClassName = "NOTEPAD"
Case "paintbrush": strClassName = "pbParent"
Case "wordpad": strClassName = "WordPadClass"
Case Else: strClassName = ""
End Select

If strClassName = "" Then
lngH = apiFindWindow(vbNullString, strAppName)
Else
lngH = apiFindWindow(strClassName, vbNullString)
End If
If lngH <> 0 Then
apiSendMessage lngH, WM_USER + 18, 0, 0
lngX = apiIsIconic(lngH)
If lngX <> 0 Then
lngTmp = apiShowWindow(lngH, SW_SHOWNORMAL)
End If
If fActivate Then
lngTmp = apiSetForegroundWindow(lngH)
End If
fIsAppRunning = True
End If
fIsAppRunning_Exit:
Exit Function
fIsAppRunning_Err:
fIsAppRunning = False
Resume fIsAppRunning_Exit
End Function

Public Sub ExportToExcel()
If fIsAppRunning("Excel", False) Then
If MsgBox("V dannij moment zapuschen Excel! Dlja prodolzhenija raboti neobxodimo ego zakritj! Zakroite Excel i nazhmite OK, inache Excel bydet zakrit bez soxranenija tekyschix dokumentov!", vbExclamation + vbOKCancel) = vbCancel Then Exit Sub
Do While fIsAppRunning("Excel")
fCloseApp ("XLMain")
Loop
End If

End Sub

Забыл сказать, что это проверено при работе с Acc97, Excel97, AccXP, ExcelXP и под Win98,2000 и XP.