Solved
It was a problem with the .htaccess file. Wordpress is installed in the root directory and there is a /stats subdirectory, in which Piwik is installed. In Wordpress I have some plugins for security which do start the rewrite engine, but don’t stop it. So to solve this error, I made another .htaccess file, which I put inside the /stats directory. in that .htaccess-file I’m only stopping the rewrite engine. Now it all works perfectly again.