Add Account (POST) Endpoint No Longer Working in new Yodlee API - yodlee

Question

OVERVIEW
I am not sure what happened, as I have not touched my code that interacts with the Add Account endpoint in Yodlee (POST /{cobrandName}/v1/providers/{providerId}), but it has become broken (it was working as of Thursday). I am now getting this reply:
{ errorCode: 'Y400',
errorMessage: 'Null argument specified',
referenceCode: 'RB_134ae91e-4c80-4ad5-aac5-243ca428421d' }
DETAILS
I have the appropriate route params inserted into this POST /{cobrandName}/v1/providers/{providerId} and am passing a stringified JSON object as the request body (per the documentation) that looks like this:
{
"provider": [{
"id": 492,
"name": "Fidelity Investments",
"loginUrl": "https://login.fidelity.com/ftgw/Fas/Fidelity/RtlCust/Login/Init?AuthRedU",
"baseUrl": "http://www.fidelity.com/",
"favicon": "https://moneycenter.ydlstatic.com/fastlink/appscenter/siteImage.fastlink.do?access_type=APPS_CENTER_PRODUCTION&siteId=492&imageType=FAVICON",
"logo": "https://moneycenter.ydlstatic.com/fastlink/appscenter/siteImage.fastlink.do?access_type=APPS_CENTER_PRODUCTION&siteId=492&imageType=LOGO",
"status": "Supported",
"mfaType": "Multiple levels of strong authentication.",
"oAuthSite": false,
"lastModified": "2016-03-31T10:39:36Z",
"forgetPasswordUrl": "http://personal.fidelity.com/accounts/services/content/pinchange.shtml.tvsr",
"containerNames": ["tax", "investment"],
"loginForm": {
"id": 172,
"forgetPasswordURL": "http://personal.fidelity.com/accounts/services/content/pinchange.shtml.tvsr",
"formType": "login",
"row": [{
"id": 4140,
"label": "Username",
"form": "0001",
"fieldRowChoice": "0001",
"field": [{
"id": 358,
"name": "LOGIN",
"maxLength": 15,
"type": "text",
"value": "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",
"isOptional": false,
"valueEditable": true
}]
}, {
"id": 4139,
"label": "Password",
"form": "0001",
"fieldRowChoice": "0002",
"field": [{
"id": 357,
"name": "PASSWORD",
"maxLength": 20,
"type": "password",
"value": "285968df9ddf385b2da03d7247533dd0f7dbf6d951c0a6a92bad7701868d4941bcdfe7a41167f3bb0a47cb5ae22557444f83a641318c1985035c6e2c7e7e11a4e71ac52965ad26845ee64a13a2a61c07945870b7c009aba673e01b074fc047e9b08c20cbde0cb36a6068a9d07b4cad7a777f8f6654dd8436527d6017e3a81af68f1dbbfabc52a93a2f8423b342b243316248cebb549bed7433f77a90aae4a3c1c892a725872f77a369407741be98715b39039b547f735e92748ddd5d389c4de3082684c1f5f5b1917ceb34348010866ce6cb649d6e4dac4c45d85c4a729c67fb643c34e704278f3a15626bbe73cddf69cfe5df8bcdd5682415afe952585da800",
"isOptional": false,
"valueEditable": true
}]
}]
}
}]
}
The form values are encrypted via PKI as per the documentation, using the key provided by the appropriate API endpoint, and I have both a cobrand session token and a yodlee user session token in the authentication headers ('Authorization': 'cobSession={appToken}, userSession={token}'). I was previously receiving back the expected 'refresh info' response using the same exact structure and process. I'm trying to understand what the 'null argument' referenced in the error is.