VBA macro not running with excel file open

Hi everyone

I am having problem with a vba code.
I design a form, coded everything and it seems to work fine. When opening the file, the form is launching and excel is hidden.
my code is:

Private Sub workbook_Open()
Application.Visible = False
FrmRapport.Show
End Sub

It all works fine if I load the file without any other excel file open.
Can anyone help me?

Thanks