This should be added as a system requirement for installing phpBB 3.2. I encountered it, similar to the solution in this post:

https://www.phpbb.com/community/viewtopic.php?f=556&t=2439031#p15070801

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.