OS : Ubuntu 14.0.4.5 LTS
PHP Version: PHP 5.5.9-1ubuntu4.22

Original warning message:

WARNING: /piwik/core/Plugin/ReportsProvider.php(166): Warning - usort(): Array was modified by the user comparison function - Piwik 3.2.0 - Please report this message in the Piwik forums: http://forum.piwik.org (please do a search first as it might have been reported already)

reported here first: https://forum.piwik.org/t/usort-warning-reportsprovider-php-166/26341