Hello @pahendriks and welcome to the forum!
Is it OK now? I didn’t do anything, but somebody (or some tool) might have fixed a web server recently.
Firefox view-source:https://updates.duplicati.com/beta/latest-installers.js was giving:
<html>
<head><title>502 Bad Gateway</title></head>
<body bgcolor="white">
<center><h1>502 Bad Gateway</h1></center>
<hr><center>nginx/1.11.5</center>
</body>
</html>
instead of its normal list of installers. updates.duplicati.com seems to do 502 Bad Gateway sometimes. There are other reports in the forum, usually on update failure, but maybe download links also degrade?