Hey Lennart,
To test, I’ve created a test page in your site and inserted your grid’s shortcode.

Nothing was displayed in the frontend.

That was because you haven’t set the post type.

I selected Inlagg and the post from that post type are shown now.

The Classic Columnize is only limited to 2 columns. You will need to use the builder and the V2 Text element. There’s currently a known issue with V2 Text Column Setup and other slide based options in the latest release though so if you’re going to use it, I’d recommend that you check out our Changelog first to see if a fix has been released.
Thanks.