TFT_eSPI: Raspberry Pi Pico with Pico-ResTouch-LCD-3.5 not working

Other Hardware Displays
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Thanks @david_prentice
Sorry for the late reply, I was on holiday.

I tried your suggestion, but I still cannot see anything on the screen.
My code seems to do something, but the screen is black.

#include <SPI.h>
#include <stdio.h>
#include <TFT_eSPI.h>             // Invoke hardware library
TFT_eSPI tft = TFT_eSPI();        // Invoke custom library

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  
  delay(3000);
  Serial.println("Hello");
  
  //initialise TFT_eSPI library
  tft.init();
  tft.begin();

  delay(1000);

}

void loop() {
  // put your main code here, to run repeatedly:
  tft.fillScreen(TFT_WHITE); //if this line is commented out, the next serial output is much faster --> something seems to happen.
  Serial.println("White?");
  delay(200);

}

I have the feeling, that the backlight might be switched off. However, I cannot figure how to switch it on. I tried changing #define TFT_BACKLIGHT_ON HIGH to #define TFT_BACKLIGHT_ON LOW in the user setup, but without any obvious effect.

Do you have any suggestion on how to turn on the backlight?

EDIT:
The backlight works properly (I found a position on the device, where I can see it). I can switch it on/off and even dim it via digitalwrite or analogwrite commands.
Accordingly, the LCD screen seems to be "off".

If I run the TFT_ReadWrite_Test.ino, I only get errors like this
Pixel value written = 2
Pixel value read = 0
ERROR ^^^^
Pixel value written = 4
Pixel value read = 0
ERROR ^^^^
Pixel value written = 8
Pixel value read = 0
ERROR ^^^^
Pixel value written = 10
Pixel value read = 0
ERROR ^^^^
(and so on)

EDIT2:
I ran the ReadUserSetup tool. The result looks wrong to me. Shouldn't there be positive PIN-numbers?

TFT_eSPI ver = 2.3.70
Processor    = Transactions = Yes
Interface    = SPI
Display driver = 9486
Display width  = 320
Display height = 480

MOSI    = GPIO -111
MISO    = GPIO -112
SCK     = GPIO -110
TFT_CS   = GPIO -1
TFT_DC   = GPIO -1
TFT_RST  = GPIO -1
TOUCH_CS = GPIO -1

TFT_BL           = GPIO -1
TFT_BACKLIGHT_ON = HIGH

Font GLCD   loaded
Font 2      loaded
Font 4      loaded
Font 6      loaded
Font 7      loaded
Font 8      loaded
Smooth font enabled

Display SPI frequency = 2.00
Touch SPI frequency   = 2.50