Hi,
First, please try to test this issue with an Excel worksheet (via File - Save & Send - Send As Attachment) and check if you can reproduce it.
In addition, please temporarily turn off your antivirus software and make sure it is not integrated with Outlook, then open Outlook in safe mode to check if the problem persists. To open Outlook in safe mode, press Windows key + R to open the Run command,
type outlook.exe /safe and press Enter.
Please let me know the result.
Best Regards,
Steve Fan
TechNet Community Support
