One other thing you could try is to re-insert the images. I moved the domain name of one of our Mautic instances recently and the Images Break when you do this. Inserting them again in the email solved this form me. In that case the wrong links where in the email itself and caused errors on my emails
If that does not solve your problem I think you’ll do well to check your Ngnix config.
Try the Ngnix Forums and check back here once you’re happy that end works as expected.