Try to config Genesis without Wizard? I haven't had any problems with fastethernet.device with it like that (Wizards are useless, so I haven't even bothered to try with it

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It might be that fastethernet.device doesn't fully cover all functions it should and that's why it fails on some queries or something

It fails with executive's dashboard here too.. but when used directly from tcp/ip stack, it works just fine.
If that doesn't work.. are you sure that you have some mem from Voodoo reserved for network card? 10/100Mbps (fastethernet.device) cards need it, 10Mbps (mediatornet.device) won't. Check VoodooMem env-setting.