The remote server returned an error: (413) Payload Too Large. .net WebClient

Question

I'm trying to download a webpage. It was working previously but now I'm getting 413's specifically mentioning 'Payload too large' (unlike what seems to be the normal error message 'Request Entity Too Large')

Here's the code; I've tried all sorts of web config changes suggested in related questions but no luck.

WebClient client = new WebClient();
client.Headers.Add("user-agent", "Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)");
var page = client.DownloadString(websiteUrl);