Developer Tool (F12) of browser shows
This content should also be served over HTTPSwarning for the site:
Permanent SEO-safe 301 redirect from HTTP to HTTPSis configured for the site under Plesk > Domains > example.com > Hosting Settings > Security.
The warning is caused by the fact that the element mentioned in the warning is loaded over HTTP, not HTTPS. However, the site is configured over HTTPS protocol.
If the URL from the warning is available over HTTPS by opening a new tab in your browser, change the URL from
https:// in the code of the site.
Otherwise, disable redirect from HTTP to HTTPS.