I AM NOT COMCAST CUSTOMER SUPPORT (probably a good thing in this case). I am a comcast customer just like you. ERROR 519 is the same as 517 or 422. I have posted two solutions and am working on two more. Click on this link and keep checking back as new solutions will be posted when they are ready.
http://forums.comcast.com/t5/XfinityTV-Website/Video-cannot-play/m-p/2115652/highlight/false#M32333