Problem fixed.
The bug si about WPML and not ACF.
– Go to Media > Library
– Open any media item
– Scroll to the bottom of the page, select “Make Media translatable” option, and then click in the “Apply” button
– Now uncheck the “Make Media translatable” option and then click in the “Apply” button again.
The solution here:
http://wpml.org/forums/topic/images-disappear-from-media-library-upon-activation-of-media-translation/
