Bismillah. I have encountered that problem. We use moodle v2.5.1. I backup a course and created 4.1 GB mbz file. I need to restore it to another category, seems like creating a duplicate course. When I tried to restore it, after long time waiting, the process have been suspended and give me a message
error/tmp_backup_directory_not_found
Something odd in my mind, when I download the file, it only 147 MB in size. This happen many times. Then, I try to backup partially, not the whole course. It just 2 GB mbz file. I try to restore it to another place and seems completed normally. Then I backup the rest and restore it, it seems normal too.
Then, I suddenly remembered that our server have allocated only 4 GB to tmp folder. I think that's probably the case. If our backup file more than this size, the file would be corrupted and could not be restored.
So, if you have the same problem, try to backup-restore partially.
That's my opinion. Happy moodeling.