Hi Marco,
The error can show for some reasons. WP download manager is a large plugin, with many modules and language files. This can cause issues with some servers when using the automatic update.
It can be happen if there is an issue with the zip file you’re uploading/downloaded. It can also be happened with your hosting as WordPress doesn’t have enough memory on the web server to unzip and upload the contents of the zip file.
Please try manually reinstalling WPDM? To do so, download the WPDM plugin here:
https://downloads.wordpress.org/plugin/download-manager.latest-stable.zip
You can do this manually by logging in to your site via FTP or Cpanel, and delete the existing WPDM plugin from plugins folder. You should then be able to reinstall the plugin manually by following the instructions here:
Download Manager
It is recommended that you make a complete backup of your website before trying this.
I hope this helps! Please let us know if you have any questions.
Thanks