Dear Colin,
The RZero error is discussed in the forum topic found here: FAST throws: Rzero must be < 1 (particularly my post dated May 25, 2011). I don’t think I can provide any more advice other than what is already stated there. And there hasn’t been any progress on the AeroDyn development that will alleviate this error. My guess is there is some problem with your model that is making it go unstable, leading to excessive blade deflection that results in the RZero error.
Best regards,