Hi mikkel.lindblom,
Are you using ACF Flexible Layout with WPML already installed?, If yes, this issue was solved in my case updating WPML to current last version.
JPM
I’m having same issue, this message is under code inspector box:
Message: Uncaught TypeError: Object [object Object] has no method ‘dialog’
File: http://piscomistral.noise.cl/wp-content/plugins/sitepress-multilingual-cms/res/js/post-edit.js?ver=3.1.3:104
At least on editing a custom field group.
Adding a new one I’m having the same issue in which I can’t add new layouts too.
EDIT: Sorry, seems this particular error is caused by WPML plugin, not sure if cause ACF Felxible Content addon error.