Visited phpBB 3.2.5 site from support forum user, and noticed the following warning was being generated on the registration page:

[phpBB Debug] PHP Warning: in file [ROOT]/phpbb/captcha/plugins/qa.php on line 87: count(): Parameter must be an array or an object that implements Countable

[phpBB Debug] PHP Warning: in file [ROOT]/phpbb/captcha/plugins/qa.php on line 104: count(): Parameter must be an array or an object that implements Countable

Code inspection of the /phpbb/captcha/plugins/qa.php module in phpBB 3.2.5 confirms that "var $question_ids" doesn't have an array() declaration for PHP 7.2+.  Reviewing other uses of "count()" in the qa.php module didn't reveal any other omissions (at least not obvious enough for me to identify).