A security token is a generic error message which could cover a few things.
Your images may be too large in size, have you tried cropping or limiting the content?
Your images may be too large dimension wise, have you tried scaling them down?
You may be attempting to load too many images into a thread/post at once, have you tried limiting the amount of images?
The human verification (Solve Media Box) is a black screen, which generates an image after about 10-15 seconds. I just tested it and it is working correctly.