"Everytime I try to power down my Arietta via shutdown -h now or halt it reboots immediately afer it shuts down. What is my mistake? Or has my board any defect? Or how to power ist down properly ?"
On the Arietta, the Shutdown line is connected on the power regulator enable so to be able to put Arietta in sleep if you use a battery for the RTC.
In order to shut down Arietta properly the right command is hence
# shutdown -h -H now
In this way the shutdown command will not lower the shutdown line and you can safely remove power when the board halts.
Without the -H the shutdown line will lower at the board halt bringing down the 3V3 voltage so you are doing a very hard reset only (useful some time).