If I remember, we just added an LL_RCC_LSCO_Disable() with an angry comment in the RTC initialization code to reset the critical bits. Not the most satisfying solution, but we haven't seen the problem since :)

Thank you all for good advice,I'm fully aware about the uninitialized struct parts, but checking the code all relevant fields were initialized ok, so I guess we're happy with that.At least we found out (most likely) what is actually causing the probl...