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