Maybe the server IP is blocked? Have you tested that? I know that RealDebrid blocks many VPS/DataCenter IPs
So you server is not a headless system but yet JDownloader can't tell your OS to open browser. Are you connecting via ssh or via VNC or how? I'm asking because of remote system and GUI.
Have you installed Browser and setup as default in your System? You can also specify path yourself via
https://support.jdownloader.org/Know...aptcha-browser