Hey @simoncoz,
The common cause of mobile menu not working is version incompatibility between X and Cornerstone. Please ensure they are always up to date. See http://theme.co/changelog.
If that does not help, then there must be something causing this in your site like third party plugins and / or customization like in the case in this thread. It might be different on your end so please open a separate thread and give us WP admin access in a Secure Note.
But before you give us your credentials, please try testing for a plugin and customization conflict. You can do this by deactivating all third party plugins and removing your customizations, and seeing if the problem remains. If it’s fixed, you’ll know a plugin or customization caused the problem, and you can narrow down which one by reactivating them one at a time.
Thanks.