Jira API Error

[James Worthington]

A user is getting the following error when attempting to create an issue through the API: 

00 Bad Request
{
"errorMessages": [
"Unrecognized field \"fields\" (Class com.atlassian.jira.rest.api.customfield.CustomFieldDefinitionJsonBean), not marked as ignorable\n at [Source: org.apache.catalina.connector.CoyoteInputStream@98f8994; line: 2, column: 16] (through reference chain: com.atlassian.jira.rest.api.customfield.CustomFieldDefinitionJsonBean[\"fields\"])"
]
}

This is the JSON that is being sent via API, scrubbed of some information:

{

    "fields": {

       "project":

       {

          "key": "CT"

       },

       "summary": "TEST",

       "description": "Test: Post Man REST API",

       "issuetype": {

          "name": "Opt In"

       },

       "labels": ["LabelName"]

   }

}

I would initially have assumed that the "fields" portion of the JSON is not properly configured. However, this same request works correctly in our QA environment. I have already confirmed that there are no conditions or required fields that would be preventing ticket creation, and that the user/password they are using has Jira access and has access to create tickets in the target project. Any assistance in decoding this error message would be appreciated. Please let me know if there is any other information I can provide.

[Adam van der Voorn _TechTime_]

Are you on Server or Data Center? What is your Jira version?

[James Worthington]

We are on Data Center version 8.20.10 in both the QA and production environments.

[James Worthington]

@Adam van der Voorn _TechTime_

I'm still not able to find any cause for this error. Can you make sense of the error considering the same method is working in the QA environment?