I want to add images, but the upload button does not work.
On june 18 the last image was uploaded.
I tried another theme and deactivated the plugins wich were updated after june 18, but that did not resolve this problem.
The takeaway from your gif is simply that clicking the Upload button does nothing, is that right?
In which case I would expect you to see errors in your browser console. Could you please check and report back.
Forms 2.4 was released on June 26 and changed how media uploads are handled to use the native WP media uploader instead of the old custom solution, so it is likely that these changes are behind the problem, but any errors will help identify the problem.
It should not have affected existing forms, but it may prove helpful to edit your existing forms and try checking and unchecking the new option "Use the WordPress Media Library manager for image, video, audio, or file fields "