Ehh, I am sorry, I did not notice that it is write-only. I just read the value and it was allways 0x8000. That is why I thought the settings is lost. Then my question is answered. Thank you for opening my eyes.
If you look at [0x4000280C], you will see the content of the shadow register. This content is 0x8000 after each reset. The RTC interface marks every written shadow register in config mode for writetrough into the real RTC registers.
