Matomo tracker heavily delayed

I have a bit of a weird case:

I got a setup with multiple sites. They all work fine, except for one: On that one site, the actual request to the php client, will only happen several minutes after the matomo.js has been loaded.

The setup is fairly boring:

<script type="text/javascript">
var _paq = window._paq = window._paq || []; _paq.push(['trackPageView']); _paq.push(['enableLinkTracking']); _paq.push(['enableHeartBeatTimer']);
(function () { 
    var u = ''; 
    _paq.push(['setTrackerUrl', u + 'matomo.php']); 
    _paq.push(['setSiteId', 'n']); 
    var d = document, g = d.createElement('script'), s = d.getElementsByTagName('script')[0];
    g.type = 'text/javascript';
    g.async = true; 
    g.src = u + 'matomo.js'; 
    s.parentNode.insertBefore(g, s); 

The matomo.js gets loaded within 1.5 sec of page load (and response fully ready about 50 ms later), but the matomo.php ping only happens 9 minutes (!) later.

Does anyone have any ideas?

Thank you!