Error after moving to the new server

[user 1-2-3]

I moved vBulletin installation 4.2.5 from old server with PHP 5.6.30 and MySQL 5.5.57-cll to the new one with PHP 7.1.11 and MySQL 5.6.38-log. Now I have the following error:
----------------------------------
Call to undefined function mb_detect_encoding() on line 729 in /home/user1/public_html/includes/functions.php
#0 /home/user1/public_html/includes/functions_forumlist.php(281): fetch_censored_text('\xD2\xE5\xE3 textarea \xE8 ...')
#1 /home/user1/public_html/includes/functions_forumlist.php(230): construct_forum_bit('1', 2, 0)
#2 /home/user1/public_html/forum.php(611): construct_forum_bit(-1)
#3 /home/user1/public_html/index.php(43): require('/home/user1/...')
#4 {main}
----------------------------------

For testing purpose I also installed a fresh installation of 4.2.5 to the same host and it work without any error.

I need help to find out how to fix above mentioned error. Thank you in advance.

[Mark.B]

You need to ask your hosts to add the mbstring library to php.