All emails show "Unscannable Attachments" banner

Question

As of last week, ALL email (attachments or no attachments, plain text & HTML) show the "Unscannable attachments" banner on emails. 
 Restarted the appliance to no avail, I have had to turn off the banner for the time being however all systems seem to be running ok. Ive requested callbacks from Sophos and attempted to get though on the phone numerous times but never can. 
 Help would be appreciated.

Draco · Accepted Answer

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.