it is a typical Speedport-Telekom Home Router. I double checked, if there is a device with the same ip-address, but there isn’t. I tried to change the addresses because i wanted the pis to be further back in the ip-address range. The pies are all at: 192.168.2.240-250.
Never mind, I will upgrade to a FritzBox soon, lets see if the problem stays after the transition.