Port forwarding forwards to nonexistent device

I'm trying to port forward to my PC at (dynamic) address 10.0.0.125. But every time I do a port forward either on your webpage or the Android app, the port is forwarded to something called "No Associated Device" at 10.0.0.3.  There is no device 10.0.0.3 on my network! A Google search shows that some other folks have had this same problem.

I rebooted the gateway and disabled advanced security but that didn't help. I spoke with a Comcast technician by phone. She had no suggestions other than replacing the Gateway. But if this is a software/firmware problem, that won't help.

Any ideas?