If available, try one of the IPSWs for an earlier version of the beta.
If anyone gets this to work
I'm back to 10.0.3, but maybe Apple pulled beta 5 for a reason? so if you can't download it, just have patience. We'll either get a new beta next week or the final 10.1 release (I hope).
Did you try downgrading to an earlier version than 10.0.3 to install the developer beta? I'm curious because I've been attempting to do this myself