Error: 'Codes 4XX and 500.19' Problems with Workflow Forms and general troubleshooting

In some cases, a custom Work flow Form (WFF) may experience errors.
Customer have just reported that the forms are not opening and you just get the loading indicator spinning around, it is usually described as a spiny thing.
Error references also dealt with in this article 401, 403, 404, 408, 413,414, Http Error 500.19