In CTS 1.6.X code, we cannot read the serial number of the GEN4 displays, which may be why you're receiving the error. In CTS 1.7.1 and higher we are able to read the serial number of the GEN4's. Did you see the same error when the unit was at 1.8.0, prior to downgrading to 1.6.7?
On the WebGUI, do you see the display serial number listed (Display_Serial, should be in the same area as the Display_Hardware model/Display version you posted a screenshot of earlier)?
In addition to Antonio's suggested output of "show upgrade detail," you should see the serial number listed in the output of "show hardware video."
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