I followed this. i could not for the life of em download any file from the backup. Each and every time I tried the system logged me out,
One tidbit to add, if you are not comfortable getting into the database, you can simple go to your file system editor in cPanel or on your hosting account so you can see the file tree structure, backup the course, then immediately go to the file system to (moodledata/filedir) and locate the absolute latest created folder. locate the latest created folder by date and time, and download that text file, then rename it to .mbz.
i have verified it works, and you can get your courses backup.
happy hunting.