@Jinxing Chen
Change into 'long hash name' directory. Then ls -l ... list long. If there is a backup.mbz file there, it's more than likely a valid backup that couldn't be 'copied' to destination. Use 'mv' on the backup.mbz to any location outside moodledata or to a file system repository in moodledata.
If you see a backup.mbz.gasee23 kinda file name, that's a temp file that was being built and it is NOT a valid/completed backup.
However, if one sees a moodle_backup.xml file there, along with other .xml files and corresponding directories, the building of the mbz could have failed. But, you could try removing the temp backup.mbz.qffwer file, then making a .mbz of your own:
tar -cvzf mybackup.mbz ./*
while in the 'long content hash' directory.
Not a 100% deal, but might be worth a shot!
Check PHP settings for time for a script to run and other such settings ... increase them. You could also turn on debugging, do a backup process, to see what debugging might offer. Definitely look at apache error logs or php error logs.
The only way to know if mybackup.mbz will work or not is to try to restore it and after the first couple of steps in restoring, cancel the operation ... assume that it is good. (fingers crossed)
'spirit of sharing', Ken