AustPost saying errors in shipping estimator

[Trinity]

I have the same message coming up on orders;

Unexpected error. No valid method. Using static rates.

I am using version Zen Cart v1.3.8a and Ozpost version V2.0.5

Has anyone experienced this and how did you resolve the problem?

Thanks

[Trinity]

I have some added information;

Have upgraded to Ozpost version V2.0.6

Shipping calculator still defaults to flat rate BUT only when logged in as a test customer (US). If I use the shipping calculator with a guest cart (using same country - US) there is no problem at all???

Thanks in advance.