by Ken Task.
One more thought in troubleshooting this ...
Be the backup cli or GUI, moodledata/temp/backup/ is used.
Failed backups will leave .log files larger than 0 bytes and contain the 'plan' steps for a backup. Failed backups will also leave a directory by a long name. cd into one of those.
You should find some .xml files which are road maps to other directories therein ... files.xml goes with the files directory.
Might take a gander at that stuff for a clue.
Focus, of course, is 'special characters'!
'SoS', Ken