Oops ... sorry bout that ... giving you in-correct command. :\
In moodledata/temp/backup/ there might be some clues. Moodle uses that directory to unzip into a hashnamed folder consisting of letters/numbers. Inside that folder are the un-zipped contents. Is there a moodle.xml (if a 1.9 course backup) or a moodle_backup.xml (if a 2 backup) present at that level. That one xml file is like a roadmap as to what to restore. There will be a log file that is larger than 0 bytes. Can open it in a text editor and inspect what it says/shows. Clues could be found in there as well.
Turning on debug will report errors to the screen. However, one might find same references in logs ... if your server is logging errors.
Do you have command line shell access to your server? If so, you could upload the .mbz to a test directory (outside of Moodle data) and via commands test your server ability to unzip.
In test folder - file is somebackup.mbz (the file you uploaded):
mv somebackup.mbz somebackup.zip
unzip somebackup.zip
You should see the files/structure (ie, the folder names, etc..) replicated as they existed on your local machine's 'test' folder.
'spirit of sharing', Ken