Xiao BLE QSPI flash P25Q16H with Zephyr / Nordic nRF Connect SDK

I´m trying to add Xiao BLE to the Zephyr based nRF Connect SDK.
My settings for the QSPI flash is not working. The system doesn´t start.
Here is the part 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.
Does anybody know what´s wrong?
To check the device I used an Arduino setup which is working as expected.

Strange, I changed sck-frequency to 104000000 (which is from the data sheet).
The system starts and I´m able to read and write the flash!
After adding some parameters like writeoc to the device tree it doesn´t work any more even if I revert my changes!