How to make a download-button from "File Upload" field?

Hello Sascha,

Meta Box is not compatible with Kadence Block yet. So I think it's not possible to add the file field URL to the image block of Kadence in this case. But you can create a custom shortcode to output the image with file URL download, please follow the documentation
https://codex.wordpress.org/Shortcode_API
https://docs.metabox.io/fields/file-upload/#template-usage

Hello,

Following the documentation, our plugins support outputting the field value with code. If you want to make a download button with a page builder plugin, please contact their support to ask for help with this case.
Or you can use a page builder plugin that has integration with Meta Box like Oxygen, Elementor ... please read more here https://docs.metabox.io/compatibility/

Or you can create your own shortcode and put the shortcode into the builder to display a download button.
https://support.metabox.io/topic/adding-a-custom-field-as-a-link-to-an-elementor-button/#post-34356