Problem processing inbound emails to activities
Yes it sounds like it's hitting an email it doesn't know how to parse - see the code for that function formatMailPart() where it fails (https://raw.githubusercontent.com/civicrm/civicrm-core/master/CRM/Utils/Mail/Incoming.php). Can you tell which email? Anything odd about it? In particular if you view the message source there might be a "Content-Type:" line with an unsupported type.

That "fix" for 4.7 all it does is just skips the email instead of a fatal error, but you'd still have an email that doesn't get processed.