PHP function putenv is disabled on some Servers by Default.

The CUSTOMIZE section in ACP cannot be opened completely.
I made a quick change in the file "root/phpbb/composer/installer.php" on line 120 as follows:  

if (function_exists('putenv'))

{

    putenv('COMPOSER_HOME=' . filesystem_helper::realpath($root_path) . '/store/composer');

} else

{

    // TODO info: putenv has been disabled for security reasons

}