Circle with question mark instead of font awesome's icons

In my case it was that i wrote invalid prefix for brands icon:

<i className="fa fa-instagram">

instead of

<i className="fab fa-instagram">

Version 4 just had one prefix — fa, but older versions have 4 prefixes.

See upgrade notes: upgrading-from-4

Everything was added. But problem was in second font-awesome script that was manualy added. After deleting this problem dissapears.