I found the reason for getting one MAC-Address only - I switched the network from "Cloud Track" to "MAC address" and now I see the 2 addresses who cause the conflict.
![SMR_0-1576853242403.png SMR_0-1576853242403.png](https://community.meraki.com/t5/image/serverpage/image-id/10978i97C8AF3725937F82/image-size/medium?v=v2&px=400)
Solving the IP conflict message is up to the vendor of the acting devices (or meraki to mark some messages as expected without sending an E-Mail out).
To stop the flood we disabled the IP conflict alert again.