And another auto-reply 
Thanks to the support, we’ve figured out the issue, it was the issue with the version.
Official image is still in 2.5, but the nextcloud app does not check and use a feature from 2.6.
To solve it, connect to your raspberry via ssh, and run a:
apt-get update && apt-get dist-upgrade
You might have a fatal error near the end related to git and etckeeper, but it’s not a real concern, the support told me etckeeper was quite tedious about having your git identity set, which is not in my case as I don’t need my raspberry pi to known about git.
Then check your version changed to 2.6, and run the nextcloud install again from the administration interface.
Hope that help someone