the Page Overlay function doesn't work on pages starting with www, if the www-Versions aren't explicitly added as second URL entry for any Website. It just shows the following message:
You are attempting to open Page Overlay for the URL "http://www.website.com/".
None of the domains from the Piwik settings matches the link.
You can add the domain as an additional URL in the settings.
The function can't find the above mentioned page because the URL version containing www isn't added for any Website, as it is not needed according to the following note on the Sites Manager:
It is recommended, but not required, to specify the various URLs, one per line, that your visitors use to access this website. Alias URLs for a website will not appear in the Referrers > Websites report. Note that it is not necessary to specify the URLs with and without 'www' as Piwik automatically considers both.
The Page Overlay function should follow this rule and also consider Websites containing the URL without www.