As a temporary solution, try and go directly to the networks website and watch from there.
There have been a number of threads talking about this:
http://forums.comcast.com/t5/XfinityTV-Website/Error-517/m-p/2059397#M31632
http://forums.comcast.com/t5/XfinityTV-Website/error-517/m-p/2108496#M32218
Not much in the way of help but you can try this https://forums.adobe.com/thread/695456 and if that doesn't help private message one of the Comcast support people here are ask them for help; please supply as many details as possible i.e. operating system; web browser; verify your Flash version; be specific about what shows you're trying to watch.
- Norm