0 */6 * * * /home/user/refresh_jd2_cookies.sh (every 6 hours).

JDownloader 2 does natively import .txt cookies. You need a workflow:

Using "premium databases," shared accounts, or "cookies.zip" files for JDownloader 2 is generally not recommended

A premium database is typically a shared list or a specific file ( org.jdownloader.settings.AccountSettings.accounts.ejs ) that contains pre-configured login credentials for various file hosters.