I managed to dump all the eight partitions from a pristine NSA325 and flash U-Boot to NSA320.
It worked and now NSA320 has a running bootloader, with probably a wrong environment.
I tried to apply the procedure described here
ftp://ftp.zyxel.it/guide/nas/nsa320_recovery_firmware.pdf to restore kernel and rootfs using this package
ftp://ftp.zyxel.it/guide/nas/nsa320_recovery_usb.zip as described many years ago here
https://archlinuxarm.org/forum/viewtopic.php?f=23&t=6532#p36351 but sadly the kernel is loaded by U-Boot then it hangs...
I also ask myself why the WarheadsSE dump didn't work...
What might be the reason the Zyxel supplied kernel does not boot? Who knows....
Anyway I am still looking for dumps from a pristine NSA320, including whole nand, so to restore it in its exact conditions: if anyone has one....