PHP Fatal error: Uncaught InvalidArgumentException: item should be a Collection or an array or an object in /wp-content/plugins/sitepress-multilingual-cms/vendor/wpml/fp/core/Obj.php:112, referer: hidden link
We decided to set the ACF_Group to Not translatable. After that we had to change a lot of settings and resave the most of our pages, but it's working now the way it should be.
This ticket is now closed. If you're a WPML client and need related help, please open a new support ticket.