I think I know what is wrong, but I do not understand why. I had to change the BOOT0 jumper to 1 for the debugger to be able to flash the code and to reset the CPU. However when I want to debug the code with the SysTick interrupt enabled, I have to change the BOOT0 jumper back to 0 before I start the code execution. Debugging works fine now, but I have no clue why. Can somebody please help me to understand this?