Your software
My Mautic version is: 5.2.8
My PHP version is: 8.3.26
My Database type and version is: MySQL 8.4.7
Your problem
My problem is:
All image URLs in the email builder are missing the protocol from the Site URL in the configuration.
This is also affecting the avatar image, which is prefixing the hostname of the container to the ?d= parameter, since UrlHelper::rel2abs thinks the avatar path is relative (since it’s missing the protocol).
So, the UI is using http://, but your proxy is on https://, right?
I still haven’t figured out how to solve this issue on Mautic, but a suggestion is to use rewrite rules in your proxy to upgrade clients from http to https.