Hi,
i found a solution thanks to this thread filesystem - How do I use OverlayFS? - Ask Ubuntu
You have to add a requires option for the overlay to wait until the desired upper
directory is ready to mount.
The additional entries in the fstab file looks like this:
/dev/disk/by-label/Settings /settings ext4 defaults 0 2
/dev/disk/by-label/Home /home ext4 defaults 0 2
overlay /etc overlay defaults,x-systemd.requires=/settings,lowerdir=/etc,upperdir=/settings/etc/upper,workdir=/settings/etc/working 0 0