This should be added as a system requirement for installing phpBB 3.2. I encountered it, similar to the solution in this post:
In certain cases the search queries worked fine but in other cases not, eventually causing a timeout error like this in the PHP error log:
17-Aug-2018 19:22:14 America/New_York] PHP Fatal error: Maximum execution time of 30 seconds exceeded in /home/moviecra/public_html/vendor/symfony/polyfill-mbstring/Mbstring.php on line 431
Enabling the PHP mbstring extension solved the problem.