Server 2003 SP2 only: Window handle disappears

[msdnuniv]

Hello!

I have an application running since 2004 on win98 to Server 2003 SP1 and
Vista without problems. But on Server 2003 with SP2 sometimes window handles
of opened, but hidden windows disappear. If i try to show the window again,
I get a
System.Reflection.TargetInvocationException: Unable to get the window handle
- the closing event of the form is _not_ called
- i still can use functions and variables on this window.
- i cannot .show() it.

Does anybody know about this or has hints how to manage this or even any
hint how to find the point where this might happen?
(Some code below)

Thanks!
Dirk

'---- code ----------------------------------------
class main
Private Shared m_frmEBrowser As e_browser

Public Shared Sub initWindows (ByVal asinterop As Boolean)
If m_frmEBrowser Is Nothing Then
'Instantiate window, write it to a local variable, so
'that any sub can get it over an property
m_frmEBrowser = New BoschSISMain.e_browser(asinterop)
End If
'...
End Sub

Public Shared ReadOnly Property win_E_Browser() As
BoschSISMain.e_browser
Get
Return m_frmEBrowser
End Get
End Property
End Class

'If i use in any other class
StartMain.win_E_Browser.Show()

'I get the exception