When my system has the default route with metrics > 0 (before launching mudfish), it seems that mudfish will attempt to retrieve setup information using its own tap0, which has no connectivity, and then will fail to setup the routes because no information is received.
I had to manually setup my default route to have metric = 0 to resolve this issue. It would be nice if mudfish can handle this situation by itself. I suggest, since it’s using dhclient at the moment, that you provide a script (man dhclient-script) to remove the default gateway added since it’s not used anyway (except maybe in vpn mode?).