This problem occurs when the site is9 accessed in the browser using HTTPS (secure) but with Moodle configured to use HTTP (not secure). The browser will block non-secure content for a secure page.
Check the $CFG->wwwroot line in config.php and make sure this matches the address being used in the browser, i.e. $CFG->wwwroot = 'https://…' if HTTPS is being used in the browser.