Your pump shutdown temperature at 60 is too high which increases the difference between the return temperature and the boiler return temperature when starting. Set there at 45 °. Set the pump flow temperature to 50 °.
You are not equipped with minimum return temperature control? If yes, put it on 55 °, if not, equip yourself with a probe that you put on the return.
Actually your RTB is always 100% ... Perhaps bad combustion, how are the ashes? Either boiler too small. Or both :-)