Typical recommendations, when this happens, are :-

- Use less connections (Max.Con.); reduce them to 1 or 2
- Check whether your download folders still exist
- Allow java.exe, javaw.exe and jDownloader in your firewall
- Unstable internet connection, contact you ISP
- Reboot your PC
- Un-install / re-install firewall
- Un-install / re-install virus scanner
- Your torrent client is using too many connections
- Un-install JRE (Java) and re-install latest JRE version

If all this doesn't solve your problem, please post a link to a complete and detailed log file.