All emails show "Unscannable Attachments" banner

Create a mirror copy of the Unscannable attachment rule above the unscannable rule and set it to quarantine messages send from one sender. As that sender send a test message. In the quarantine you should be able to see the details of the message and see what attachments it is triggering on.

As this is all email, it could be an image file attached in a signature. The image may have been corrupted in some way which prevents the engine from scanning the whole file correctly.

There could be many other reasons for this issue, but I hope this guides you in the right direction.

Create a mirror copy of the Unscannable attachment rule above the unscannable rule and set it to quarantine messages send from one sender. As that sender send a test message. In the quarantine you should be able to see the details of the message and see what attachments it is triggering on.

As this is all email, it could be an image file attached in a signature. The image may have been corrupted in some way which prevents the engine from scanning the whole file correctly.

There could be many other reasons for this issue, but I hope this guides you in the right direction.

This seems like the best course of action. Quarantine the messages and see what could be unscannable. Another thing to note is if this is happening for inbound or outbound mail.