In the namespaced file install/convert/convert.php, constructor for the class convertor is defined in 'old' style

https://github.com/phpbb/phpbb/blob/3.2.x/phpBB/install/convert/convert.php#L56

As per PHP manual http://php.net/manual/en/language.oop5.decon.php

As of PHP 5.3.3, methods with the same name as the last element of a namespaced class name will no longer be treated as constructor. This change doesn't affect non-namespaced classes.

So, the constructor doesn't work, which leads to errors like

Fatal error: Call to a member function error() on null in /install/convertors/functions_phpbb20.php on line 1348

See https://www.phpbb.com/community/viewtopic.php?f=556&t=2434161