I was having this issue for almost a month then I found a solution by going to my Router web site (ATT.com). I have a Motorola NVG510 Router. On my T.V., I went to Network Settings, then Network Setup, then Network Services. I noticed a setting that said "Automatic" and below that were two blank spaces, one was marked Stb and one was marked Audio-so, I changed that to "Manual" and the two blank spaces were filled with five digits each.
Also, make a note of your IP Address-it should start with 192. etc, etc. You will need that later. Also you may need Stb and Audio numbers ( I didn't but I made note of them anyway). Once you make those changes, you will be directed to open your Router's configuration page (it sounds scary but not really). You just go to the address line and enter 192.168.1.254. You will be asked for your Routers' Device Access Code which can be found near the bottom of your Router.
Once there, you will need to access your computers firewall, from there you will need to locate and access the NetGaming section, then you will be asked to "Add a Port". My device was already listed in the drop-down menu-so I just clicked "Add" then Save. I was also asked for my IP Address which I had recorded earlier - I added that and hit Save again. That was it.
I then went back to my TV and went to Network Setup and repeated that step and it indicated that the connection was successful, from there, I went to On Demand and selected a show to "watch now" and no more error message.
Hopefully, this will help others who were receiving the error message mentioned in this thread.