Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/STMicroelectronics/STM32F105xx/BKP/RTCCR#0x0
RTC clock calibration register (BKP_RTCCR)
Calibration value
Calibration Clock Output
https://github.com/cmsis-svd/cmsis-svd-data