First of all, please note that I have split the existing thread to create a new one on your behalf as it is recommended to share any distinct problem in a new thread for better visibility and tracking.
Regarding your original concerns, please check the following.
Does the referenced sheet2 actually contains any Pivot Tables? Before assigning it to another object please check the size of the collection.
Are you using same versions of Aspose.Cells for .NET API on both local and server machines?