Thank you JesusGon,
But the new version of wpForo comes with Font Awesome 5. Font Awesome 5 are deprecating “fa” class. All icons come with fas, far, fab… classes. There should be another solution for this icon, which will be compatible with FA 5 future changes.
BTW, you can fix it very easy. Please put this CSS code in Dashboard > Forums > Settings > Styles > Custom CSS Code textarea. Save it, delete all caches and do Ctrl+F5 on forum front.