by Séverin Terrier.
Hi Ken,
and thanks for your answer and suggestion.
For what i've seen, some other courses containing same signs work well.
But i've discovered that all courses in error (in automatic backup) have a shortname over a certain value, and thus a backup filename over a certain value, which seems to break the backup processus (now).
I'll have to do more tests to confirm the value.
And then, the need to understand what changes between Moodle 3.3.4 and 3.3.5 which make backups impossible.
Séverin