Hmmm, see my thinking.
PC is connected to router ethernet interface, so most likely they should be on the same subnet. You perform 2 pings, one sourced from the ethernet interface, and the other sourced from the LAN interface.
The one sourced from the LAN interface does not reply, but the serial does. There is no fancy config, no policy base routing on the router, hence the forward path for both pings should be the same, since the destination is the same. I therefore assume that in both situations the packet get to the PC.
I will also assume that the PC will reply to both, since there are no firewalls. But the destination ip are now different, and hence the return path could be different. The router recieves the packet for the serial but not for the ethernet ip. This will imply that the reverse path for the ethernet ip is not ok. This however contradicts the fact that from the PC, pings were going to the router ethernet interface.
The only thing I can think of is that the replies to those routers ethernet interface were coming from another rogue device. Which will also explain the reason why normal pings from the router was not going through. A proxy-arp scenario could still allow pings to go through to and from the serial interface.
Theory, theory, theory. I hope the original poster can shed more light on the practical situation.
