This is a software bug. And this could happen if a device has an IP outside DHCP address range for current network.
Some possible actions that could lead to this
- DHCP addresss range was changed(at either upstream router or Firewalla box running in DHCP mode) after some device already got an IP assigned
- A device has a static IP configured , which is outside DHCP address range for current network
Before the fix is released, a quick workaround would simply delete such device in App.