I am using an Apex trigger and asynchronous method to perform a callout to the JSON REST API for Wordpress. I continue to receive 500 Internal Server Error responses in my debug log when I edit a Salesforce record, but if I copy/paste the corresponding JSON into the Postman plug-in for Chrome the post is created as expected. Any help is appreciated, thanks!

trigger insertPost on Custom_Object__c (after insert, after update) {
    String theIds = '';
    for (Custom_Object__c c : Trigger.new) {
        if (theIds == '')
            theIds = c.Id;
        else
            theIds += '|' + c.Id;
    }
    //Make asynchronous web service callout to insert post
    if (theIds != '')
        WebServiceCallout.sendPost(theIds);
}

public class WebServiceCallout {
 
    @future (callout=true)
    public static void sendPost(String theIds) {
 
        HttpRequest req = new HttpRequest();
        HttpResponse res = new HttpResponse();
        Http http = new Http();
        
        if(Test.isRunningTest()){
            Website_Settings__c wpSettings = new Website_Settings__c(Wordpress_URL__c = 'https://testing.com/', Wordpress_Username__c = 'Test', Wordpress_Password__c = 'Test');        
            insert wpSettings;
        }
        
        //Set Headers from Custom Settings & Visualforce
        Website_Settings__c wpSettings = Website_Settings__c.getInstance();
        if (wpSettings.Wordpress_URL__c!=null && wpSettings.Wordpress_Username__c!=null && wpSettings.Wordpress_Password__c!=null){
            
            //Set Endpoint
            String endpoint = wpSettings.Wordpress_URL__c;
            if (endpoint.endsWith('/'))
                req.setEndpoint(endpoint+'wp-json.php/posts/');
            else
                req.setEndpoint(endpoint+'/wp-json.php/posts/');
            
            //Set Method
            req.setMethod('POST');
            
            //Specify the required user name and password to access the endpoint
            String username = wpSettings.Wordpress_Username__c;
            String password = wpSettings.Wordpress_Password__c;
            Blob headerValue = Blob.valueOf(username + ':' + password);
            String authorizationHeader = 'Basic ' + EncodingUtil.base64Encode(headerValue);
            req.setHeader('Authorization', authorizationHeader);
            
            //Specify Content-Type
            req.setHeader('Content-Type', 'application/json');
            
            //Specify Cache-Control
            req.setHeader('Cache-Control', 'no-cache');
            
            //Set Body
            objectToJSON.theId = theIds;
            objectToJSON jsonClass = new objectToJSON();
            req.setBody(jsonClass.jsonData());
            
            try {
                System.debug(req.getHeader('Authorization'));
                System.debug(req.getHeader('Content-Type'));
                System.debug(req.getBody());
                if (!Test.isRunningTest())    
                    res = http.send(req);
            }
            
            catch(System.CalloutException e) {
                System.debug('Callout error: '+ e);
                System.debug(res.toString());
                System.debug(req.getBody());
            }
        
        }
 
    }
 
}

public with sharing class objectToJSON {
    //Global variables
    public static String theId;
    public list<String> theIds;
    public list<jsonObjectData> allObjects{get;set;}
    
    public objectToJSON() { 
        //***This method queries the database for information and converts it to JSON***
        //If URL parameters are set then query the database
        if (ApexPages.currentPage()!=null)
            if (ApexPages.currentPage().getParameters().get('theId')!=null && ApexPages.currentPage().getParameters().get('theId')!='')
                theId = String.escapeSingleQuotes(ApexPages.currentPage().getParameters().get('theId'));
        if (theId.contains('|'))
            theIds = theId.split('\\|',-1);
        else{
            theIds = new list<String>();
            theIds.add(theId);
        }
        allObjects = new list<jsonObjectData>();
        list<Custom_Object__c> objects = [SELECT Id, Name, Description__c, A__c, B__c, C__c FROM Custom_Object__c WHERE Id IN :theIds LIMIT 10000];
        for (Custom_Object__c o : objects){
            allObjects.add(new jsonObjectData(o));          
        }
    }
    
    public String jsonData() {
        String jsonData = '[';
        Integer x=0;
        while (allObjects.size()>x) {
            if (x==0)
                jsonData += '{';
            else
                jsonData += ',{';
            jsonObjectData o = allObjects[x];
            jsonData += '\"title\":\"'+o.name.replace('\"','\\"')+'\",';
            jsonData += '\"status\":\"publish\",';
            jsonData += '\"type\":\"custom\",';
            jsonData += '\"content_raw\":\"'+o.content.replace('\"','\\"').replace('\n','')+'\",';
            jsonData += '\"excerpt_raw\":\"'+o.excerpt.replace('\"','\\"').replace('\n','')+'\",';
            jsonData += '\"comment_status\":\"closed\"';
            jsonData += '}';
            x++;
        }
        jsonData += ']';
        if (x==1){
            //Remove array formatting for individual post
            jsonData = jsonData.substring(1);
            jsonData = jsonData.substring(0,jsonData.length()-1);
        }
        return jsonData;
    }   
     
    public class jsonObjectData {
        //*****Wrapper Class*****
        
        //Global variables for wrapper class
        public String name{get;set;}
        public String content{get;set;}
        public String excerpt{get;set;}
        
        public jsonObjectData(Custom_Object__c o) {
            //***Constructor method to create a JSON object***
            
            //Define content variables
            String a = '';
            String b = '';
            String c = '';
            
            //Set content variables
            if (o.A__c!=null)
                a = String.valueOf(o.A__c);
            if (o.B__c!=null)
                b = String.valueOf(o.B__c);    
            if (o.C__c!=null)
                c = String.valueOf(o.C__c);

            //Define & Set description variable
            String description = '';
            if (o.Description__c!=null)
                description = String.valueOf(o.Description__c);
            
            //Set name
            name = o.Name;

            //Set content
            content = '<div class="custom"></div>\n<div class="row clearfix ">\n<div class="col col_1_2 ">\n<div class="inner">\n<div class="styled_table table_blue">\n<table>\n<thead>\n<tr>\n<th>Custom</th>\n<th> </th>\n</tr>\n</thead>\n<tbody>\n<tr>\n<td>A</td>\n<td>'+a+'</td>\n</tr>\n<tr>\n<td>B</td>\n<td>'+b+'</td>\n</tr>\n<tr>\n<td>C</td>\n<td>'+c+'</td>\n</tr>\n</tbody>\n</table>\n</div>\n</div>\n</div>\n</div>\n';

            //Set excerpt
            excerpt = '<p>'+description+'</p>\n';
        
        }               
    
    }
    
}