Thanks @Aleks to look at my issue!
cat /etc/yunohost/settings.json displays nothing 
For your information, I had an issue with normal system upgrade because of sudden full root partition.
I deleted some “fat” logs (Nextcloud and Synapse mainly), and gained around 4-5 Gb and now my root partition is fine again (yunohost system diagnosis confirms everything is fine).
In Kimsufi debian install, root partition is not that big (18Gb) so I have to be careful.
After cleaning system log, system upgrade functions normally again.
But then I had Nextcloud that remained in maintenance mode after a failed upgrade (see [Nextcloud] A cloud for personal or group use - #419 by croulibri).
I solved that issue of maintenance mode (Nextcloud is online again), but upgrade of Nextcloud is still not possible because of failed backup process.
Could it be that my now solved situation of full root partition (only root was full, /home partition have more than 200Gb free) is the root of all these problems?
One day everything will be fine again 