Hello,
I had a message on the GUI saying :
Erreur: "500" Internal Server Error
Action: "PUT" /yunohost/api/apps/nextcloud/upgrade
And after in the logs
Backup failed, the upgrade process was aborted.
I solve it following your tips 
Thank You 
Hello,
I had a message on the GUI saying :
Erreur: "500" Internal Server Error
Action: "PUT" /yunohost/api/apps/nextcloud/upgrade
And after in the logs
Backup failed, the upgrade process was aborted.
I solve it following your tips 
Thank You 