NAS326 not discovered by NAS seeker after firmware update 5.21

I had problems connecting my NAS326 to an AURENDER music server (it worked fine over the last 4 years). The NAS was normally discovered by the NAS Seeker, then I opened the web interface and proceeded with a firmware update (5.21). Now the NAS is NOT discovered by the NAS seeker.

NAS is visible in the network, but it tells "no contents shared".

Tried telnet to the IP address visible in the network, but connection is refused.

Also tried hard reset with the reset button, no success.

Help is highly appreciated!

Thanks!