by Ken Task.
The trick is to make sure when you re-zip (before changing the extension back to .mbz) you zip the contents of the extracted files ... not the folder that the original unzipping may have created. Moodle must find moodle_backup.xml at the 'root' of the zip when it extracts ... otherwise, you get the error you have reported.
Ok, ... example:
somebackup.mbz
mv somebackup.mbz somebackup.zip
unzip somebackup.zip
That creates a folder called 'somebackup' and contained therein are the extracted files.
cd somebackup
do whatever changes ... when done
and from the somebackup folder location:
zip -r somebackup.zip *
mv somebackup.zip somebackup.mbz
Use the .mbz for restore.
'spirit of sharing', Ken