There is an issue opening reports generated by Courier in Excel 2016.
After speaking with Salesforce, we've determined the issue is due to a Microsoft update.
What To Do
To work around this issue, Microsoft has released a patch that allows you to open files exported using Export Details, the method Conga uses to deliver Courier reports. Salesforce is continuing to work with Microsoft to resolve the issue.
Review this Microsoft knowledge base article for more information and links to install the Microsoft patch. We recommend reviewing this information with your IT department prior to installing.
Please see the following known issue from Salesforce documenting this issue: https://success.salesforce.com/issues_view?id=a1p3A0000008fb
Another workaround is to change the export type in Courier to a CSV or HTML file, or display the report inline with the email.
We apologize for any inconvenience this may cause.