A:Answer I have a similar setup. I have not gotten this error. Is the ROKU giving the error message? If so, hook up the ROKU directly to the HDMI on the 3700 and see if you get the error. Make sure the ROKU is set up to 1080p and not 4K, by hooking it up to a different TV. If the message is from the 3700, where the ROKU is hooked up through your receiver, make sure the Onkyo is set to 1080p video if it has that ability. Usually, it should choose this automatically. I have also heard of this error happening with bad HDMI cables in reports online for 4K especially, or for times when the length of cable is too long or there is cheap HDMI cable used.
These are some things you can use to diagnose, but I'm not entirely sure of the issue. It definitely is a video problem. Something isn't passing on your video OR your 3700 is having problems with HDMI and needs to be returned. AGain, hooking the ROKU and a DVD player etc to the 3700 directly should tell you if the 3700 has a bad Video port. If it does, it won't show video. Also, did you hook up the HDMI to the normal HDMI port on the 3700? There are 2 ports, but the instructions suggest hooking it to one normally (not the MHC or whatever its called). THese may help you to diagnose a few things. Hope it helps. PS.... as you are doing tests, use a different HDMI cable and one as short as possible. I moved all my receiver and roku and dvd player to near the 3700 to not have HDMI length video problems. D