We are facing the same issue for JExcelApiExporter and JRXlsExporter both on JR5.5.1 and POI3.11. The xls files don't open in Excel but opens in Openoffice/Libreoffice. Excel Shows unreadable content, and when clicked yes it says document correpted.
I tried using JRXlsxExporter also. Using that the file still shows unreadable content, but when clicked yes it says hyperlinks removed.