If I make a new project, enable HSE, SWD and USB, compile and flash it, USB works fine. If I then proceed to also enable LSE and use it for RTC, USB breaks. I make literally no other changes than just enabling LSE and setting it as the RTC-source in clock configuration and yes, all the HSE-related clocks remain the same as they were before. Simply removing LSE and setting LSI for the RTC gets the USB going again!
I am a beginner, just trying to learn how to work with these STM-parts and software and I'm uncertain if I'm just missing something or if this is an actual bug.
