On July 12, 2016, Microsoft released a security update. As a result of this Microsoft security update FIMS CRM users might not be able to export their reports using the “Printable View” or “Export Details” buttons.
FIMS CRM is functioning correctly and this is strictly a Microsoft issue. Microsoft published a KB article (https://support.microsoft.com/en-us/kb/3181507), which includes suggested workarounds. At this moment we are unsure of if/when Microsoft will fix this issue.

From the MS KB article above:
For option 1, you have the two options for the format of the export (xls or csv (this one seems to open fine)).
For option 2, after the export is downloaded but before opening this file, do these steps outlined in option 2.