Mixed content error on Emails Record View

When opening the Record view for an Email (v8.0.1) with an embedded image I get an error:

Mixed Content: The page at 'https://sugarcrm.mydomain.com/sugarcrm/#Emails/33241198-e2d7-11e8-8a99-001a4a160206' was loaded over HTTPS, but requested an insecure image 'cid:33243434-e2d7-11e8-92d0-001a4a160206.png'. This content should also be served over HTTPS.

There are no customizations that would affect that view.

Any thoughts on how to resolve this?

thank you,
FrancescaS