Error while Proxying Request

When I attempt to PATCH a resume upload, I receive a

{
"errors": [
{
"errorMessage": "Error while Proxying Request",
"errorCode": 0
}
]
}

error response. I've double checked my token, and I've tested other failure conditions, which give the expected failure messages, so it's not a error proxying the request on my side, otherwise the message wouldn't change when I introduce errors.