Got some success 
This what I did :
- mount the xfs drive with
context=system_u:object_r:container_file_t:s0 - init migration
- go into the app environment and
# Fix permissions
# 1. inside the container :
grep ^nextcloud2: /etc/subuid
nextcloud2:493216:65536
# 2. fix from outside the container
chown -R 493298:493298 /mnt/nextcloudstorage # # 493216+82 = 493298 !
# EDIT : that should be 493216 + 82 -1 = 493297... but it works anyway (probably not needed)
chmod 777 /mnt/nextcloudstorage # not sure it's needed
- stop rsync-nextcloudx
- delete then recreate the volume bind-mounted
podman volume create \
--opt type=none \
--opt device=/mnt/nextcloudstorage \
--opt o=bind \
nextcloud-app-data
- relaunch rsync-nextcloudx with
--security-opt label=disable! - Launch the sync
- edit
systemctl --user edit --full nextcloud-app→add --security-opt label=disable - not sure I restarted rsync-nextcloudx at this step. Might be.
- Finish the migration → success
