In the manual it states, that reset value is 0x000.., and that 0xFFFF... is reserved. But when debugging, i see, that it is 0xFF... . When now assigning a new value to it via HAL_GPIO_Init(), the register does not take the new values.
Hold on!
I just found the solution while writing this post:
Copied the code from another project, but forgot the part where the clock of that Port was activated!
