Within the last 2-3 days, all our Office Insider users have started receiving a warning pop-up when sending an email with any type of attachment. It doesn't matter if it is an Office document, PDF, Zip, or even a screen capture added to the body of the message. Additionally, we've seen this across 3 different builds, so it doesn't appear to be build-specific/related.
The message in question is:
The users can easily get around it by clicking Yes, but this has become quite the inconvenience, and furthermore there is indication where this is stemming from. I have found this article (https://support.office.com/en-gb/article/You-receive-an-error-when-opening-attachments-in-Outlook-3f6dbffd-8505-492d-b19f-b3b89369ed9b?ui=en-US&rs=en-GB&ad=GB) which is in reference to KB 3191932 (June 13 security updates), but the issue is slightly different than what is noted.
Has any other Office Insiders experienced this, or have any more information/details on what is causing it?