Just to conclude this thread: I have found and downloaded the 1.34 Firmware. There is a windows installer, which can be found on the web. (Drivers.softpedia.com). The installer is Japanese only, but Google Translate on the iPhone did the trick. I directly connected the LS421DE to a Windows Laptop with a crossover cable. It took the installer two retries to find the LS421DE, but after that I could easily install the older firmware. I DID NOT loose my settings and data on the LS421DE, but of course I had made a backup before. So far the NAS works. I will post in a few days, whether the connection still is working and the LS421DE does not freeze.
Regards,
Rainer