The ExportFieldValue.aspx control has been updated with additional security enhancements.
During migration, the ExportFieldValue.ascx.cs (.vb) file in your application is replaced with an updated file. Any changes you have made to this file will be lost. This file is located in:
...\<Application Folder>\Shared\ExportFieldValue.ascx.cs (.vb)
After migration, re-apply any changes you may have made to the original ExportFieldValue.aspx.cs (.vb) file to the new file in your application.
Version 5.1 Application Migration