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This sounds like a firmware related issue not a hardware one, most likely the AV box has received an update which is now checking for copy right protection or the encoding has changed on the input boxes (DVD and cable).
In the first instance I would look to check all 3 devices have the latest update applied, some cheaper brands of HDMI are known to cause issues with data transmission, which may be why the AV cant authenticate and verify the products are not copyright protected, however as you have already tried another cable this sounds unlikely.
Ill stand by, and wait to hear back from you.
This sounds like a firmware fault, unfortunately the only other thing I can suggest is doing a factory reset, failing that it will need to have the firmware flashed which can only be done at a service centre.
The process to perform a factory reset is defined on page 40 of the manual here.