Similar issue here. My box suddenly stopped working with my Sony STR-DN860 receiver. Displaying black screen only. Welcome message comes up on reboot, then nothing. Attempted rebooting several times, multiple calls to Comcast tech support, 2 tech visits. 2nd tech finally mentioned that this is a know issue with 4K boxes and some receivers. Comcast has no known fix so far. Switching the box HDMI cable from the receiver to the TV (Sony Bravia XBR-65X800B) solved the issue. So cable box now outputs to receiver via TV’s HDMI ARC port rather than directly. Not an ideal solution but seems to work for the moment. Appears that Comcast’s last firmware update broke things, as the box has worked fine with a direct receiver connection since it was installed a year or so ago.
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