Can't read GPIO registers (yes, I enabled GPIO clock)

I am trying to control the GPIOs of DWM1000 but the read operation for example of register GPIO DIR returns 0xDEAD01FF.

The bytes on the wire are exactly:

MOSI 66 08 00 00 00 00
MISO 7F 00 FF 01 AD DE

I can communicate reliably with the module. Chip ID is correct (0xDECA0130). I can control other registers. I can read/write PMSC registers without problems. SPI clock is 2 MHz. Tracks are very short. Signal integrity looks okay.

I am using code based on ex_10a_gpio and decadriver:

if (dwt_initialise(DWT_LOADNONE) == DWT_ERROR) {
    printf("INIT FAILED");
    while (1) {}

dwt_setgpiodirection(DWT_GxM0 | DWT_GxM1 | DWT_GxM2, DWT_GxP0 | DWT_GxP1 | DWT_GxP2);
dwt_setgpiovalue(DWT_GxM0, DWT_GxP0);  // no voltage appears on the pin
dwt_setgpiovalue(DWT_GxM1, DWT_GxP1);  // no voltage appears on the pin
dwt_setgpiovalue(DWT_GxM2, DWT_GxP2);  // no voltage appears on the pin
dwt_setgpiovalue(DWT_GxM3, DWT_GxP3);  // no voltage appears on the pin

printf("reg dir %08X", dwt_read32bitoffsetreg(GPIO_CTRL_ID, GPIO_DIR_OFFSET));  // prints 0xDEAD01FF
printf("reg raw %08X", dwt_read32bitoffsetreg(GPIO_CTRL_ID, GPIO_RAW_OFFSET));  // prints 0xDE000000

while (1) {}

How can I debug this issue?
Thanks for help