Amanda:
I'm sorry but this post is not very helpful, we need way more details in order to help you.
Are you getting this error on every show or just specific shows?
What web browser and operating system are you using?
Try and watch directly from the networks own web site, are there any problems doing that or does playback work fine?
Check that your Flash version is the latest http://www.adobe.com/software/flash/about/
Try this https://forums.adobe.com/thread/695456
Try and use the Adobe Flash uninstaller http://helpx.adobe.com/flash-player/kb/uninstall-flash-player-windows.html to uninstall and reinstall Flash.
If you are using Internet Explorer, try FireFox or Chrome.
If all else fails send a private message to one of the Comcast support people; provide as much information about your problem as possible; what's shows you're having problems with; that streaming works from the network site but fails on the Xfinity site (or not); what browsers you've tried; what version of Flash you're using; every piece of data could help point to the problem.
- Norm