We're unable to process live transactions using USA ePay.
Here's what USA ePay says about this error code:
The merchant has set a pin for this transaction but the api did not receive a UMmd5hash. They need to either upgrade their software to send the hash or they need to remove the pin on the source.
We confirmed that our Source Key is correct, and I'm working to gain access to the clients USA ePay dashboard to poke around, but thought I'd pass this by you to see if you've come across it before.