call to tracking code blocks webserver (no replies)

Hi,
after upgrading from version 2.5.0 to 2.6.1 (automatic update) I realized some problems if a user entered the site I want to track. Piwik is located under another domain than the tracked site.

If a user entered the tracked site and the tracking url is called, piwik does not respond. The call seems to hang forever.
There is no load on the webserver where piwik is located, webserver/php process does noting. This call blocks the webserver for all other calls, clients are waiting for responses from webserver forever. I have to restart the webserver manually.
Even if I entered the image tracking url manually in the browser (piwik.php?idsite=1&rec=1), the problem is the same.
The strange thing is, sometimes its working but most of the time not.

After piwik update there were no errors in the log. If tracking url is called, there are also no errors.
I also tried a fresh and clean install of piwik 2.6.1 and the problem still exists (also tried beta 2.7).
For piwik I'm using debian wheezy with lighttpd 1.4.31, PHP 5.5.16 and mysql 5.5.38.
The dashboard of piwik itself works as expected, no problems there.

Any ideas what the problem might be?

regards
Alex