Visual Basic for Applications Reference
Form already displayed; can't show modally (Error 400)
Specifics
You can't use the Show method to display a visible form as modal. This error has the following cause and solution:
You tried to use Show, with the styleargument set to 1 vbModal, on an already visible form.
Use either the Unload statement or the Hide method on the form before trying to show it as a modal form.