Wrong gateway IP assumed for dynamic GW
I have been using a recent 2.0 build (04-20-2010? Not sure since I'm not at the box right now), and there seems to be a gw problem.
My configuration is: WAN (pppoE VLAN7) for internet connectivity and Opt1 (DHCP VLAN8) for IPTV on the same physical interface. For some reason, the dynamic Default GW is always the Opt1 IP (which is merely there for IGMP traffic). Whenever I manually set a default GW with the WAN-VLAN7-IP, the WAN-Vlan7-IP is immediately lost and a new one will be obtained, so the newly created Default GW will be rejected because the IP is in the wrong subnet. My fault?
Anything from April is not recent, there have probably been 400-500 changes since then. There was an input validation bug a while back on dynamic gateways, which sounds like what you're seeing. Upgrade.