How to diagnose why the Geeetech A10x display shows nothing

In Marlin the display is configured by defining YHCB2004 and it resolves to HD44780 handled by marlinui_HD44780.cpp.

For Klipper I have it defined in the printer.cfg as:

hd44780_protocol_init: True
line_length: 20
lcd_type: hd44780
rs_pin: PD1
e_pin: PH0
d4_pin: PH1
d5_pin: PD0
d6_pin: PE3
d7_pin: PC1
encoder_pins: ^PL7, ^PG1
click_pin: ^!PD2

I have double checked with the schematic for the board that the pin-out is correct. The display is plugged in with a short ribbon cable that appears to work fine with Marlin. In Klipper the display back-light lights up and the display shows the first and third rows as just boxes with the second and fourth row just blank.

How can I diagnose what needs to be changed to get Klipper to init the display successfully? This is using the current stock v4.1b control board for the Geeetech A10x (and A20x) machines.

Thanks for any advise

you solved the Problem with the display? I have the same setup…
thank u

Unfortunately, no. Nor any advise about how to diagnose why it does not work.

I did hear that the display does work on an older 4.0 board but I have the 4.1b board that is presumably the same pinout.

If you have ideas about how to figure it out or have a working solution I would be most interested!