Xiao BLE QSPI flash P25Q16H with Nordic nRF Connect SDK

I´m trying to add Xiao BLE (nRF52840) to the nRF Connect SDK.

Here is the configuration part for the flash that I use in the device tree file:

&qspi {
  status = "okay";
  sck-pin = <21>;
  io-pins = <20>, <24>, <22>, <23>;
  csn-pins = <25>;
  p25q16h: p25q16h@0 {
    compatible = "nordic,qspi-nor";
    reg = <0>;
    sck-frequency = <8000000>;
    label = "P25Q16H";
    jedec-id = [85 60 15];
    size = <16777216>;
    has-dpd;
    t-enter-dpd = <3000>;
    t-exit-dpd = <8000>;
  };
};

To simplify the configuration I used a lot of default settings. The flash should work in slowest mode.

I don´t have a debugger connected, because there are only small pads on the board which are difficult to connect to.
But I enabled UART over USB to get debug output.

But the system doesn´t start. There is no output.
It worked only once after I changed sck-frequency to 104000000 (which is from the data sheet).
After adding some parameters like writeoc to the device tree it doesn´t work any more even if I revert my changes!
To check the device I used an Arduino setup which is working as expected.