eCommerce Analytics return Bad Request

We have a problem when using eCommerce Analytics. Currently we get a BadRequest back in the browser console.


<!-- Matomo -->
<script type="text/javascript">
  var _paq = window._paq = window._paq || [];
  /* tracker methods like "setCustomDimension" should be called before "trackPageView" */
  (function() {
    var u="";
    _paq.push(['setTrackerUrl', u+'matomo.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.src='//'; s.parentNode.insertBefore(g,s);

  _paq.push(["addEcommerceItem", "123", "Testitem #1", ["Testveranstaltung #1"], 0.99, 4]);
  _paq.push(["trackEcommerceOrder", "JHFEIOUHN", 3.96]);
<!-- End Matomo Code -->


What are we doing wrong? Floats and integers are sent correctly as far as I can see?