In the middle of 2016, Microsoft released a security update in which effected the way MS Excel handles these types of files. As a result of this Microsoft security update, FIMS CRM users might not be able to export & open into Excel 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 solutions. Please try this to resolve the error.

If you have Office 2016 then use this link: https://support.microsoft.com/en-us/kb/3115438