I would say this is an Incredimail quirk.
Incredimail and clean notes are a nightmare even without trying to configure it for SSL email. This forum is littered with people having problems inserting clean notes into incredimail. The way incredimail multi-part mime with HTML and Text parts to the email, avast has to do two clean notes one for HTML and one for the Text parts of the email. Incredimail seems to have problems with this where the email appears malformed with a mix of text amd html.
Personally I see little value in the insertion of clean notes. I don't take any notice of them (trusting sod aren't I) and neither does avast it still scans your email, the same is true of other AVs, they just do what they should, scan the email. There are even spam and malicious email that has a fabrication of a clean note from a well known AV, just to lull people into a false sense of security.