Unable to backup : ExternalFolder NotFoundException


I configured External Storage app for some local folder, and configured the Backup app to use it.
But when I try to do a manual backup (because, of course, automated one never worked), I got this error :

`sudo -u www-data php occ backup:point:create -vvv

maintenance mode: on
initialization of the AppData
maintenance mode: off

In ExternalFolderService.php line 354:


The destination folder exists and the permissions are correct. It is correctly selected in the Backup app.
What can I do ?