I do see this Undefined array key "ID" in /var/www/astage.arteries.hu/wp-content/plugins/acfml/classes/Strings/Traversable/Entity.php though and this one points to our plugin, even though this is just a warning it should not be there
2) Can you please share steps on how can we see the issue and if the issue still happens, when using WP Default theme and only WPML plugins and ACF enabled?
- Please make a backup before proceeding.
- Go to ACF > Tools and export all of your field groups
- Delete all of your field groups from ACF
- Empty them from the trash
- Go to ACF > Tools and Re-import field groups
I have tried the same on a copy of your website, and it fixes the issue. Just make sure to have a backup if something goes wrong.