Visual Basic Reference
Can't show non-modal form when modal form is displayed (Error 401)
When a modal form is displayed, you cant display another non-modal form. This error has the following cause and solution:
You tried to use the Show method or set the Visible property to True on a non-modal form when another form is already displayed as modal.
Use either the Unload statement or the Hide method on the modal form before attempting to use the Show method on a non-modal form.