It's a new feature in 3.7 and off by default.
Here's another one of those '80's things' ... or any GUI interface you have into server outside of Moodle that can browse files.
Hints/clues might be in your moodledata/temp/backup/
Is a d45986723ed5486d32e2784696ed45ef.log file present?
Is it larger than 0 bytes? If so, view it's contents.
Have a 3.7.1+ (Build: 20190816) system with only one large course (24+Meg) and tested this new feature. Finished ... no error ... and the .log file for it showed 0 bytes ... nothing in it. But yours might.
Also ... by chance do you have automated backups turned on?
Too bad you don't like 80's ... one could test backing up same course from command line script in moodlecode/admin/cli/ ... backup.php ... with options to save that backup outside of the sea of files in moodledata/filedir/
If you stay on the screen where it shows progress rather than clicking away does it eventually completed? or is that what you did ... stayed ... it showed 100% in red and also failed. Wonder if it was really close but just timed out on the very last step of backup ...
The very last thing the backup routine does ... any method/means ... is copy the .mbz file in moodledata/temp/backup/asdfadfasdfad dir to whereever the destination is to be. In your case am imagining for your system that would be the sea of files in moodledata/filedir/
What, if anything, is remaining in your moodledata/temp/backup/asdfadfasdfad dir? If you see a .mbz file there ... system could not copy.
How large is that file?
No errors in apache error logs?
If you browse restore in that course is there a backup already there so you can get an idea of how large the course backup might be?
'SoS' ... a don't mind 80's guy!