R4 APIs (Millennium)

Workflow or API calls:

I'm working through creating a pdf DocumentReference at https://fhir-ehr-code.cerner.com/r4/ec2458f2-1e24-41c8-b71b-0e701af7583d/DocumentReference.

I've been correcting problems with my call by utilizing the diagnostic messages (most recently, period wanting an end only is the start and end are the same). However now I'm getting a generic 400s.

400 Bad Request
URL: https://fhir-ehr-code.cerner.com/r4/ec2458f2-1e24-41c8-b71b-0e701af7583d/DocumentReference
{
   "resourceType": "OperationOutcome",
   "issue": [
       {
           "severity": "error",
           "code": "invalid",
           "details": {
               "text": "Invalid request"
           }
       }
   ]
}

x-request-id "3b3c919f-f5ca-454a-a962-299c0b356277+4hKU_OtFF"

opc-request-id "/97A4D9CC86B14033AA97074675C57ECE/11A0DAE186CCEE4D4E62674BF144C9FA"

Here's my POST body:

{ 
    "resourceType": "DocumentReference", 
    "status": "current", 
    "docStatus": "final", 
    "type": { 
        "coding": [ 
            { 
                "system": "http://loinc.org", 
                "code": "18748-4", 
                "display": "Imaging Study Report" 
            } 
        ], 
        "text": "Imaging Study Report" 
    }, 
    "content": [ 
        { 
            "attachment": { 
            "contentType": "application/pdf", 
            "data": "<<<Base64 PDF HERE>>>", 
            "title": "imaging study" 
            } 
        } 
    ], 
    "subject": { 
        "reference": "Patient/12724066" 
    }, 
    "author": [ 
        { 
            "reference": "Practitioner/12742069" 
        } 
    ], 
    "context": { 
            "encounter": [ 
            { 
                "reference": "Encounter/97953477" 
            } 
        ], 
        "period": { 
            "end": "2023-10-05T17:15:13.486Z" 
        } 
    } 
}

How can I get more details on the problem?

If I add the following to period I get a useful message.

            "start": "2023-10-05T17:15:13.486Z" 

Thanks.