Piwik script fails on most but not all pages

Who can help me with the following strange issue:

Some pages are measured by Piwik, but most pages are not measured by Piwik. Piwik always worked fine and for some reason just stopped working on some pages. Due to aN update of the server, an update of Wordpress or an update Piwik?

Maybe you added some javascript on these pages that are not tracked anymore?
When you visit these pages maybe you see a JS error in your browser console?