Try to debug your submit process and check debug information.
Is your process built in from apex or you have written it before?
My report and form were built using the Apex tool wizards. By process of elimination I realized that the error was coming from the 'display only' field I had put on the form for one of the file uploads that I did not want a user to change. So I rebuilt the form/report so the interactive report just shows the viewable downloads (uploaded previously) by other users and the Form only shows the appropriate field for uploading new files. I get no more errors.
Thanks all for replies!