SharePoint CSOM Upload File Access Denied Error

Simple process, using a .Net Web App, create a local file and upload it to SharePoint. I thought the service account I was using had ampel permissions to the Site, but for it didnt… For testing, I granted the seveice account Full Control at the Web App level (user policy), site collection admin, and more. Nothing worked.

Sample of the code I was using:

            ClientContext ctxH = new ClientContext(hURL);
            Web siteH = ctxH.Web;
            ctxH.Load(siteH);
            ctxH.ExecuteQuery();

            List _library = siteH.Lists.GetByTitle("Drop Off Library");

            Folder _oFolder = siteH.GetFolderByServerRelativeUrl(siteH.ServerRelativeUrl.TrimEnd('/') + "/" + "DropOffLibrary");
            ctxH.Load(_oFolder);
            ctxH.ExecuteQuery();

            FileStream fileStream = System.IO.File.OpenRead(fileName);
            FileCreationInformation fileInfo = new FileCreationInformation();
            fileInfo.ContentStream = fileStream;
            fileInfo.Overwrite = true;
            fileInfo.Url = destFileName;

            Microsoft.SharePoint.Client.File _oFile = _oFolder.Files.Add(fileInfo);
            ctxHub.Load(_oFile);
            ctxHub.ExecuteQuery();

Quick and simple fix:
Grant your account / service account Design permissions to the Site/Web where you are uploading files.

Error at the web level:
C:\Windows\TEMP\tmp28E2.tmp: Access denied. You do not have permission to perform this action or access this resource.

ULS errors:
Permission check failed. Asking for 0x00040002, have 0x1B00C0310EF
Access denied.
System.UnauthorizedAccessException: Access denied., StackTrace:
Exception : System.UnauthorizedAccessException: Access denied.
Exception occured in scope Microsoft.SharePoint.SPFileCollection.Add. Exception=System.UnauthorizedAccessException: Access denied.
Original error: System.UnauthorizedAccessException: Access denied.
SocialRESTExceptionProcessingHandler.DoServerExceptionProcessing – SharePoint Server Exception [System.UnauthorizedAccessException: Access denied.
Throw UnauthorizedAccessException instead of SPUtilityInternal.Send401 for client.svc request.