I think it was bad luck.
I tried it as I did always, simple editing the IP-address and then save / save / save and apply
Device unreachable!
Could not regain access to the device after applying the configuration changes. You might need to reconnect if you modified network related settings such as the IP address or wireless security credentials.
I clicked "apply unchecked" and now I get:
root@OpenWrt:~# grep -r "192.168.178.2" /etc/
/etc/config/network: option ipaddr '192.168.178.2'