I first connected the ADxL345 to the MCU without the ADuM2401 and I got good readings (from the ADxL).
Than, with the same firmware I added the ADuM between the MCU and the ADxL235 (while supplying two different power supply to the ADuM and connecting both VDDIO and VS of ADxL to 3.3V of the isolated side). The CS of the ADxL is isolated with optocuopler.
after adding the optocoupler and the ADuM I checked that all the SPI signals looks fine (at the input of ADxL) but I don't get reading from the ADxL.