Error Log:

thrown in [...]/public_html/wp-content/plugins/wpforo/wpf-includes/functions.php on line 775
[18-Jun-2018 07:09:34 UTC] PHP Fatal error:  Uncaught Error: Call to undefined function mb_strtoupper() in [...]/public_html/wp-content/plugins/wpforo/wpf-includes/functions.php:775
Stack trace:
#0 [...]/public_html/wp-content/plugins/wpforo/wpf-includes/class-phrases.php(16): wpforo_phrase('About Me', false)
#1 [...]/public_html/wp-content/plugins/wpforo/wpforo.php(187): wpForoPhrase->init()
#2 [...]/public_html/wp-includes/class-wp-hook.php(286): wpForo->init('')
#3 [...]/public_html/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(NULL, Array)
#4 [...]/public_html/wp-includes/plugin.php(453): WP_Hook->do_action(Array)
#5 [...]/public_html/wp-settings.php(450): do_action('init')
#6 [...]/public_html/wp-config.php(103): require_once('[...]/...')
#7 [...]/public_html/wp-load.php(37): require_once('[...]/...')
#8 [...]/public_html/wp-blog-header.php(13): require_once('[...]/...')
#9 [...]/public_html/index.php(17): requi in [...]/public_html/wp-content/plugins/wpforo/wpf-includes/functions.php on line 775

Solution:

line 774:

if( 'en_US' != $locale && function_exists('mb_strlen') && function_exists('mb_strtoupper') && mb_strlen($phrase) != strlen($phrase) ) {

Hope it helps!