Anspress 4 – activating “Dynamic Avatar” give fatal error

On fresh install of Anspress 4, when trying activating “Dynamic Avatar” in “Add-ons” section, backend & frontend become blank.

I even cannot access disable it.

UPDATE:

This is from debug.log:

[01-Mar-2017 05:52:11 UTC] PHP Fatal error:  Uncaught Error: Call to undefined function mb_convert_encoding() in /var/www/dua.com/wp-content/plugins/anspress-master/addons/free/avatar.php:319
Stack trace:
#0 /var/www/dua.com/wp-content/plugins/anspress-master/addons/free/avatar.php(416): AnsPress_Avatar->generate()
#1 /var/www/dua.com/wp-content/plugins/anspress-master/addons/free/avatar.php(113): ap_generate_avatar(1)
#2 /var/www/dua.com/wp-includes/class-wp-hook.php(298): AnsPress_Avatar_Hook::get_avatar(Array, 1)
#3 /var/www/dua.com/wp-includes/plugin.php(203): WP_Hook->apply_filters(Array, Array)
#4 /var/www/dua.com/wp-includes/link-template.php(3913): apply_filters('pre_get_avatar_...', Array, 1)
#5 /var/www/dua.com/wp-includes/link-template.php(3799): get_avatar_data(1, Array)
#6 /var/www/dua.com/wp-includes/pluggable.php(2370): get_avatar_url(1, Array)
#7 /var/www/dua.com/wp-includes/admin-bar.php(258): get_avatar(1, 64)
#8 /var/www/dua.com/wp-includes/class-wp-hook.php(298): wp_admin_bar_my_account_menu(Object(WP_Admin_Bar))
#9 /var/www/dua.com/ in /var/www/dua.com/wp-content/plugins/anspress-master/addons/free/avatar.php on line 319