There is a problem with the Media Form Field in frontend forms like the "Add new article" form. When opening the Media modal box, for example to add an intro image to an article, the backdrop is in top of the modal box and therefore the content of the modal can not be used. See screenshot.

I solved now the problem temporarely by adding the following CSS to the custom CSS of the template:

.modal-backdrop {

  pointer-events: none;

}

This way I can still use the modal even if it is behind the backdrop but its defenitely not the a nice way. The backdrop should be placed behind the modal.