When Moodle courses are backed up, Moodle uses moodledata/temp/backup folder to build the backup .mbz file. Moodle must be able to create temporary folders and xml files in that backup folder in order to build the backup .mbz file (which is really a .zip).
Check ownership/permissions on moodledata folder:
chown apache:apache /var/www/moodledata -R
if on a CentOS/RHEL/Fedora system. If on 'other' use whatever appropriate.
While inspecting moodledata/temp/backup/ folder might look to see if there are any .log files there. They are text and can be viewed with cat. Sometimes they give hints. If a backup is successful one might see 0 byte .log files there.
Run the backup again. If it again fails, turn on debugging and see what it says.
'spirit of sharing', Ken
