No Visit Data is shown for a site

I set up a website and a Piwik instance at the same domain on an OS X server box. Both work with the same self signed server certificate (SSL). The Piwik instance is a sub-directory of the main site.
Both web sites work well.
I created a new site for the domain of the main site.
I inserted the provided js code for the site above the tag in the html of every page on my main site, so that it calls piwik.php to do the statistics for the main site.
But there are no visits logged in the Piwik instance.
I read the Piwik doesn’t track any visits and pages, and shows “There is no data for this report” in all reports. an the Does the Piwik Tracking tag work on https (SSL) pages? article.
My cert is self signed and thus untrusted but in my case I see, that https://url/my/piwik/piwik.php?rec=1&idsite=1 returns no errors and for every page view of my main site the correct piwik.php is called with a bunch of parameters (firebug).
There is no error logged on the console of the server when accessing piwik.php of the piwik installation (but the access is logged).
What can I do more?

I have the same issue on Matomo 3.5.1. I am tracking 2 sites using bulk import via One site has low traffic and it nicely shows up everyday. The other site is having very high traffic (400 events/seconds) and shows no events.