STM32F303 Readout Protection Level 1 - programs do not start at all
Silly thing. After setting the protection level Power On Reset is needed. No other reset type resets the micro. After the power on reset everything works fine. (only one POR needed of course after the protection level change).

It is in the RM - I have not noticed it


  If the read protection is set while the debugger is still connected
  through JTAG/SWD, apply a POR (power-on reset) instead of a system
  reset.