yep, the adblocker was the problem!
thank you very much
okay, i turned off the permanent redirect in the apache configuration, turned on the allowoveride all and put the following htaccess in the piwik root folder
RewriteEngine On
RewriteCond %{REMOTE_ADDR} !12\.34\.56\.78
RewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
RewriteCond %{REMOTE_ADDR} 12\.34\.56\.78
RewriteCond %{HTTPS} on
RewriteRule (.*) http://%{HTTP_HOST}%{REQUEST_URI} [R=301,L]
12\.34\.56\.78 = the server ip, wordpress is running on
when the user access the piwik page, he/she is forced to use ssl, and the wp plugin communicates through port 80. however, not the best solution but still one.
