impossible to upload image on frontend form

Hi,

I have a custom post type that uses Advanced custom fields and the front end form Plugin. I added two frontend forms to my pages (Archive and single), edit post & new post. in both forms it is impossible to upload an image. The corresponding button and description is correctly shown, but clicking on the button won’t do anything. If I create a new post in the backend including an image I can edit the image on the frontend form, but if I delete the image in the front end ‘edit’ form it is impossible again to upload a new image.

Any ideas??