Hi this is my first post, I have the same problem as you, I am using the 2.8ms version.
I have a usps user name, zip code, already entered under the admin, and for password you can enter anything as stated by usps. Thus, when testing checkout. I get the following error:
"An error occured with the USPS shipping calculations.
If you prefer to use USPS as your shipping method, please contact the store owner."
Atleast I know i am not the only one getting this message.
Please help us.