No HTTPS tracking with Piwik 3.0.2

After reading a lot of topics with no solutions I want to describe my problem:

I have a simple configuration:
On one webspace (shared hosting) are two domains:

I created a letsencrypt certificate and forwarded every HTTP traffic to HTTPS via htaccess:
RewriteCond %{HTTP_HOST} .
RewriteCond %{HTTPS} off
RewriteRule ^(.*)$$1 [R=301,L]

That means that I forward every traffic to the (main) domain name

At this webspace is the web content as well as the Piwik installation in a subfolder (
Piwik is also accessible via HTTPS like the rest of the content.

With HTTP everything worked fine for years.
After activating the HTTPS with the above cited forarding nothing is tracked.

I checked the tracking code and everything and found no mistake.

My tracking code (compressed javascript + image):
<script type="text/javascript">var _paq = _paq || []; _paq.push(['trackPageView']); _paq.push(['enableLinkTracking']); (function() { var u=""; _paq.push(['setTrackerUrl', u+'piwik.php']); _paq.push(['setSiteId', '1']); var d=document, g=d.createElement('script'), s=d.getElementsByTagName('script')[0]; g.type='text/javascript'; g.async=true; g.defer=true; g.src=u+'piwik.js'; s.parentNode.insertBefore(g,s); })();</script>

<noscript><img src="" style="border:0" alt="" /></noscript>

I think this is the most simple configuration imaginable…
What’s wrong…?

I have activated “force_ssl =1” and also installed and activated the Force SSL plugin - with no success, of course.

Best regards,