I am running Excel 2010/Windows 7 on a large network. I've recently updated an application that was originally written 4+ years ago and has been enhanced throughout the years. The latest version works fine on my machine, but some users are getting the following error upon opening the macro:

Run-time error '32809'
Application-defined or object-defined error

The trigger is the following code in sheet 'ThisWorkbook'

Private Sub Workbook_Open()
Application.ScreenUpdating = False
Sheets("Instructions").Activate
Application.ScreenUpdating = True
End Sub

The error is pointing to the "Instructions" sheet (in this case it is also Sheet33

Any suggestions as to why this is happening?

Art