Hey @Wilja,
I see that it’s working when you’re logged in and not when logged out. That happens maybe because your caching / performance plugin does not cache and minify resources when you’re logged in. I was not able to prove this because I could no longer access your WP Admin maybe because you changed the credentials as I was investigating.
The issue in the front-end is this.

The blog masonry script which is in VIEWS/GLOBAL/_SCRIPT-ISOTOPE-INDEX.PHP (not in the JS files listed previously) requires jQuery to be loaded in the head because it is loaded in the body part. Because of your optimization (grouping and minification), jQuery (combined and minified along with other JS) now is loaded at the very bottom of the page.

Please load the jQuery in the head and see if the issue persists. You might need to contact the caching / performance developer for this.
Also note that this is not an issue with X, though there is a performance benefit loading all JS in the footer, generally, it is not feasible because some jQuery functionality is needed to be used in the body. That is true not only for X but for many other plugins as well.
Thanks.