The problem is dual gateways in /etc/interfaces.

ip route shows

default via 172.DDD.DDD.DDDdev eth0 onlink

which means the system will try to access the internet through this interface. Not the way it should be as eth1 is your WAN link. Default route should point to the next-hop router behind eth1. As eth0 is in your internal network, it's in the same network with the rest of the devices so gateway isn't needed. All devices in the same subnet can communicate directly with each other. Gateway router is needed only to forward traffic into and out of a subnet.

Comment out or delete the the gateway in eth0 config and you should be good.

The problem is dual gateways in /etc/interfaces.

ip route shows

default via 172.DDD.DDD.DDDdev eth0 onlink

which means the system will try to access the internet through this interface. Not the way it should be as eth1 is your WAN link. Default route should point to the next-hop router behind eth1. 

As eth0 is in your internal network, it should be in the same network with the rest of the devices so gateway isn't needed. All devices in the same subnet can communicate directly with each other. Gateway router is needed only to forward traffic into and out of a subnet.

Comment out or delete the the gateway in eth0 config and you should be good.

If you still get same message, give commands ip addr flush dev eth0 and ip addr flush dev eth1, and retry ifup.