Blank screen when reusing a previous mail (Mosaico)
Using the browser network tools shows a failure to load the template with a &quot;mixed content&quot; message.  The problem is that the template metadata includes the full URL of the template.
The fix is to update the template URL in the Mosaico metadata to use https:// in place of http://
With db access, you can update template_options in civicrm_mailing.
Alternatively, start from cv api Mailing.get id=...
Update
There is now an API call to update the templates for you.  See the docs
cv api MosaicoTemplate.replaceurls from_url=&quot;http://old.server.org&quot; to_url=&quot;https://new.server.org&quot;