If you've tried Running 4k video downloader as admin, uninstalled and reinstalled the app, ran the app in smart mode, changed the download destination, and confirmed that no firewall/antivirus is blocking access, or entirely turned off firewalls, rebooted your device. Changed proxy/VPN, then you should probably update your operating system and scan your entire device for virus infection. You should also contact a 4k video downloader from their official website to confirm the issue isn't from their website and support the website/server you are trying to download videos from.