I wrote a tuto for that but in french
But, in anycase, the "why" it's doing that is a little bit weird.
From what I've heard, it may be cause by the TV or the handshake didn't make it... Some TVHD doesn't really support HDCP completly. (I've heard Toshiba a lot)
From what I can say on my side..when I get a case like that, everytime, I just reset the box, turn off the tv, wait for the time to come back in front of the box, re-open the box, wait about 30 sec and re-open the tv. If it's not working, just try to close the box and your tv, open your tv and re-open the box.
Something more rare, if you still have that error, it's the close the tv and box. Remove the power cord and HDMI cable from your tv, do a reset of the box, wait 3min, replug the HDMI and replug the power cord. Open the box, wait 30 sec, open your tv and hope it will be working!
If still, that doesn't work... and btw, I'm not 100% sure, but I've heard they are working on a fix for that trouble but don't know when it will be out. But again, we should be able to downgrade the OS... But... don't call and said that out loud, it will do nothing, it's just in rare case that they can do that. From the beginning of that trouble, I've just heard 2 person that have been downgrade... soo... don't hope they'll do that right away. They will more blame it on your tv, and it's a little bit true.
Also.. while waiting for a patch, you can plug a Component cable.
Videotron doesn't provide HDMI-only connection and will not warranty you that HDMI cable will work 100%. So, even if you yell at us, we provide you a cable service and it's working, it's just an imcompatibilty trouble with the box and your tv and you can plug it with a component cable and it will work good.
So, hope a fix will be out really soon, I'm gonna say it when we will have a fix!
And with that, you shall plug component cable