If you get an error that the Email Attachment Body is Null you may need to have a System Administrator check that you don't have High assurance session required for exporting and printing reports. The CSV, XLS, and Inline endpoints do not support high assurance session settings, so you can either change your access policies or switch to XLSX format if you have Report Sender Plus.

To change the access policies, read on.

Solution:
Disable the High assurance session required option in order to be able to use Report Sender. This is under the Identity Verification menu in Setup.