When one of my sites had this error, it was because students were using an old non-secure HTTP link to access a page in a course, which conflicted with the secure HTTPS link to FontAwesome that our Boost theme used.
If you're using Boost or another default Moodle theme that uses FontAwesome icons (e.g. in the menu bar), you'll need to encrypt your site with an SSL certificate and forward all HTTP traffic to HTTPS.