This error has been illusive in that it appears from time to time.
However, question for investigation ... in your moodledata/temp/backup/ directory (in Windows which I think you are using the slashes lean the other way) ... see if there is a long directory name consisting of a bunch of letters/numbers. Change into that one and see what's there. IF you see a backup.mbz file there along with a moodle_backup.xml you got close. Last action is to **copy** the .mbz file to it's destination ... which by default settings is /moodledata/filedir/SEAOFFILES ... none humanly recognizable.
So check ownerships/permission on moodledata/filedir/ as well as moodledata/temp/backup/ as Moodle needs to destroy what it used to build a backup.
It could also be a time out issue ... see your Moodle's Server -> PHP info and the settings for time for script to run, memory for a script to use, etc. Might need to bump those values upwards a little in your php.ini file. Restart web service if you do that.
Courses have a tendency to grow rather than shrink thus backups/restores need more resources from server.
How about I stop there .... and let's see if any progress is made. IF not, might have to attempt a 'deep dive'. :\
'spirit of sharing', Ken