Domain Name resolving does not work after sysupgrade

I just made a sysupgrade to openwrt version 21.02.2 on my Netgear Nighthawk X4S R7800 and after successfully upgrading and restoring the configs backup all connected clients can’t resolve domain names anymore. When I do a ping to on the router itself everything works just fine, but when I do the same on a connected client I get “unknown host”. But when I try a ping to on the same client everything works just fine. It worked right before the sysupgrade with version 19 of openwrt.

Thanks in advance for any help on this!

what DNSes do the clients get ?