Hello again and thank you for your help.
I updated the system to version RUT2_R_00.07.01.4 but this did not resolve the issue.
Cleared last buggy entries in firewall file and could get the zones back.
Clearing the network settings with your code:
uci delete network.lan2.type && uci set network.lan2.ifname='@lan' && uci commit
and initiating a restart, finally did the job.
I can now ping all adresses in both networks trough the router, even without creating firewall settings!
Perfect!
Thank you a lot for your help.
Followup question:
A device from 172.16.8.0/24 is scanning the 192.168.8.0/24 network, but can't find a modbus-tcp device (192.168.8.71). Is there a special protocol necessary wich does not get trough the (bridge)?