Authentication not longer working after update 2.2.3

Hi, I used the authentication: https://getkirby.com/docs/solutions/authentication

And it worked perfectly until I updated.

Now I keep getting

Warning: session_start(): Cannot send session cache limiter - headers already sent

It now seems to have a problem with:

<?php if(!$site->user()) go('/') ?>

If I remove this piece, everything works. But the page is unprotected as well then…
Also, if I paste this code snippet into other templates to protect them, the same error will occur, disregarding the content, as long there is a controller used!

What could be the problem? Were there big changes in the latest updates?