Which version of the Toolset are you running? I just successfully downloaded the running config from a 3750 with no error. I'm running 10.5.
Also, have you verified your credentials are correct?
I'm running Version 10.5.0.79 and my SNMP credentials are correct. Otherwise I would get another Error Message.
This problem appears everytime when I connect to one of our Switches (2960 / 3750).
Please open a support ticket and reference this thwack post.