Finally got it. Checked the browser, and saw this:
<html>
<head><title>413 Request Entity Too Large</title></head>
<body>
<center><h1>413 Request Entity Too Large</h1></center>
<hr><center>nginx/1.18.0</center>
</body>
</html>
NGINX? Yes, I have an NGINX (reverse-)proxy between the website and the interwebs.
Thought that was not important.. Sorry to waste everyone’s time!
Note to self: Check the WHOLE chain.. ??