this.

plus this.

because this.

but first this, because when you do test mode (hold func while power is coming on and then follow prompts) after the error test, you can start pressing the keys and on the screen it will say what button you pressed. When you turn an encoder it will say which encoder, and when you press an encoder button it will say the encoder button is pressed.

So first press the encoder C button so that you can see how it registers the button press on the screen. Then turn encoder D so it registers that encoder D was turned. Then press encoder D, if it registers that encoder D button was pressed, then there is a problem which is not just the physical button unable to be pressed. If it doesn’t register the encoder as being pressed at all, the button function may be broken or something, and then refer back to this:

Let us know how the test works out though.