Not possible to add media to the media browser in combination with the contrib insert module

Problem/Motivation

It is not possible to add media to the media browser. The submit button will stay hidden after selection of the selected media item.

Steps to reproduce

I installed media_directories, media_directories_editor and media_directories_ui and use seven as Administration theme.
Go to the mediabrowser (/admin/content/browser) and click 'Add Media'. Then chose the combined upload. After clicking 'Browse' select an image and click 'Open' and return to the 'Add media / Select files' popup. Result: the Submit button is still hidden. The only option is to push the cross.
See attached image.

Proposed resolution

Our form needs to be revised against the one copied form core.

Remaining tasks

Find the issue with the insert module.