Piwik is a downloadable, open source (GPL licensed) real time web analytics software program. It provides you with detailed reports on your website visitors, the search engines and keywords they use, the language they speak, your popular pages and much more. Unlike remote-hosted web analytics services (like Google Analytics), Piwik is installed on your own server. The statistical data obtained is stored in a MySQL database and allows you full control over the statistics.
Piwik has trouble with generating stats with some CMS applications such as vBulletin.
The issue appears to be caused by conflicting JS files.
JS files are used in the tracking code that you add on the pages you wish to receive stats for.
A solution seems to be to use "literal" tags. You can read about this here: