Email-to-activity processing - Error message: Process Activities failed
So after plenty of back and forth, the issue seems to be the fact I am specifying a Source folder in my IMAP account.

When I created a brand new account for testing, found same issue with a folder named CRM. Tried crm in case there was case sensitivity, still the same.

Only when I removed source and let it read from the Inbox directory did it work consistently. It was nothing to do with a specific email, simply the folder being used. 

Would love to know if this can be fixed (or even where in the code it is so I can try myself) as meanwhile I will need to maintain an extra email account.