Well, it looks really strange as you got the issue for very few users. I am afraid, we need a sample demo project, zip it and post it here to reproduce the issue on our end.
Also, which version of the product your are using. We also recommend you to try our latest version i.e. Aspose.Cells for .NET (Latest Version)
Here is a sample project that I have created. There is a somewhat unusual structure that mimics what we are doing in the actual application. Default.aspx includes a UserControl called CoursePage.ascx, and CoursePage.ascx includes a UserControl called FileViewer.ascx.
Will you please check if the error is occurring on certain browsers like firefox, IE etc. Also mention the browser versions being used by your clients. If it is IE and later than 7.0, is it being used in compatibility view etc.