Unfortunately, Piwik is not compatible with the Apache web server module
mod_security. If your web host uses mod_security to block requests containing URLs (eg. hosts like HostGator, The Planet), you should contact your provider to have your Piwik application whitelisted and have mod_security disabled for Piwik.
With mod_security enabled Piwik will look like it works but mod_security has and will create major bugs in Piwik, including major data loss as requests are discarded by mod_security! For more information, see this ticket on Piwik issue tracker.