We have followed all of the troubleshooting steps in the link you provided — we have flushed permalinks, ensured the correct WordPress rules are in the .htaccess file and can confirm that mod_rewrite is enabled on our server and this website’s account. The rest of the site is working fine, the 404 errors are only coming from the WP Download Manager plugin.
The 404 errors for this package are all referencing your plugin’s cache folder e.g. /wp-content/uploads/wpdm-cache/1601253451wpdm_P1050949-48×48.JPG
We have cleared the plugin’s cache and can confirm that files with matching names exist in the /wp-content/uploads/download-manager-files/ folder.
Can you please explain why your plugin is generating all of these 404 errors from the wpdm-cache folder, and why we can’t save changes to the “Select Members” option?