Unable to update a multi user picker using a REST API

Jira Development
1

I am getting the error “{“errorMessages”:[“Internal server error”],“errors”:{}}” when using a REST API to update a multi user picker.

The JSON string is: string jsonString = @"{ ""fields"": { ""customfield_10400[0]"": ""JDoe""}}";

I have also tried:
string jsonString = @"{ ""update"": { ""customfield_10400[0]"": [{""set"":{""key"": ""jdoe"", ""name"": ""JDoe"" } } ] }}";

and:
string jsonString = @"{ ""update"": { ""customfield_10400[0]"": [{""add"": {""id"": 10300, ""name"": ""JDoe""} } ] } }";

The code is:

objJira2.JiraUrl2 = objAssignee.Key;
objJira2.JiraUrl = System.Configuration.ConfigurationManager.AppSettings["JiraUrl"];
objJira2.JiraUserName = System.Configuration.ConfigurationManager.AppSettings["JiraUserName"];
objJira2.JiraPassword = System.Configuration.ConfigurationManager.AppSettings["JiraPassword"];
objJira2.JiraJson = jsonString;
objJira2.updateJiraIssue(objJira2);

public String updateJiraIssue(object objJira2)
            {
            JiraService.open("PUT", JiraUrl + "rest/api/2/issue/" + JiraUrl2);
            JiraService.setRequestHeader("Content-Type", "application/json");
            JiraService.setRequestHeader("Accept", "application/json");
            JiraService.setRequestHeader("X-Atlassian-Token", "nocheck");
            JiraService.setRequestHeader("Authorization", "Basic " + GetEncodedCredentials());

            try
                {
                JiraService.send(JiraJson);
                }
            catch { }

            String response = JiraService.responseText;
            JiraService.abort();
            return response;
            }
2

I have a JSON-formatted file with the detail for the error I am getting. How can I attach it to this issue or send it to Atlassian?