Yes, that was it. After upgrading I can assign the SSH permission. Nice warning about attack surface, no manual migration needed.
Now it works!
FACL gave me some trouble; first I did
setfacl -dRm g:sshfsnc:rwx /home/yunohost.app/nextcloud/data/wbk/files/
for all my user’s data, but it turns out that the group sshfsnc I made in the admin interface, is not propagated as a system group (or membership is not propagated).
After setting FACL for the specific user (recursively behind …/wbk/files, directory by directory for the parent path), I could cd to the files.
setfacl -Rm u:wbknc:rwx /home/yunohost.app/nextcloud/data/
Take note: future ACL’s are applied by using -dRm, but current ACL’s are not applied by changing the default (with -d), so you need to use -dRm as well:
setfacl -dRm u:wbknc:rwx /home/yunohost.app/nextcloud/data/
Thanks a lot!