Solved
Asynchronous Prescaler RTC STM32F103xx cannot be set higher than 127 in STM32CubeMX
The STM32F103 does not have a Synchronous Prescaler. and the Asynchronous Prescaler can be set to (2^20 - 1) in the MCU.
But STM32CubeMX doesn't allow a number higher than 127. Not really a problem of course, just a bit confusing.