How can I flush the DNS cache of a USG LITE 60AX?

Lohkamp

I had a client connected once directly via WLAN to the WAN gateway, an AVM FRITZ!Box which is now again connected to the Zyxel WLAN.

But nslookup of this client still lets the USG report [client].fritz.box with its old IPv6.
This is a show stopper, I expect I will have to reboot the USG LITE 60AX which breaks current internet connections.

For sure I rebooted my current other Windows client from which I issued the nslookup.