I first got this error with FISS, but on trying to troubleshoot via the swagger documentation at api.firecloud.org, I found it persisted:
returns:
{
"statusCode": 500,
"source": "FireCloud",
"timestamp": 1527875295457,
"causes": [],
"stackTrace": [],
"message": "sendReceive doesn't support chunked responses, try sendTo instead"
}