Need more information. What OS is the media server, what version of NetBackup? Has this always happened? All backups to this media server result in it hanging? What type of backup - filesystem, database, snapshot?
"But it's ok when I manually backup." Is the manual backup with or without BMR? What happens if you run a backup without BMR?
What NBU logs are enabled on the media server?
Anything useful in /var/log/messages?
What other processes are running on the media server at or shortly after midnight?