Mixed content on an HTTPS page
What it means
These HTTPS pages reference images, stylesheets or scripts over plain HTTP. Browsers block active mixed content outright and warn about passive mixed content, so the page is both broken and insecure.
How to fix it
Change every http:// reference on the page to https://, or make it relative by starting it at the path. Most cases come from full http:// addresses typed into old content or in a theme. Once the references are fixed, add a Content-Security-Policy: upgrade-insecure-requests header so any that are missed are upgraded rather than blocked.
Finding it in CrawlStat
After a crawl, open Issues and pick Mixed content on an HTTPS page. Click Show all in Pages to filter the Pages table to the affected pages, or Export affected to hand the list to whoever owns the fix. Audit ID: mixed_content.