SSO redirect loop

I can even reproduce with this page:

<!doctype html>
<html>
  <head><title>foo</title></head>
  <body>
    <a href="https://forum.our.tld/t/title/3940/551">https://forum.our.tld/t/title/3940/551</a>
  </body>
</html>

When clicking on the link I’m in the redirect loop, if I copy/paste it it works.
I’m … puzzeled.
The only thing I can think of that’s different in a browser when clicking on a link vs. copy/pasting the link is the referrer.