Hello, @mathieu Fontaine,
1/ Do you have the same notice about the image’s lazy load when all 3rd party plugins are disabled? Maybe, some settings, of a plugin on your site, caused the issue, please check.
2/ Can you please share your site wp-admin URL and access in the private content area and allow us to debug your site in order to solve your issue.
Note: Please take a complete backup of your site first then share the access of your site.
Regards 8Themes Team.