Hi @pierreozoux,
I just updated the Richdocuments/Collabora Online App in Nextcloud from 1.1.7 to 1.1.9, whic fixed the bug for me. Awesome, now Collabora Online works again in my Nextcloud installation.
By the way, I still use the Collabora Online Docker container and I don’t use aufs as storage driver. In Debian Stretch (with Linux kernel 4.7), AFAIK Docker uses the overlay storage driver per default. And apparently that works well with the Collabora Online Docker container.
Probably this topic should be closed now that the issue has been resolved?