Ferroli Domina F24E boiler, stops (block) after couple of seconds...

Hi everyone and thanks for any input in advance. I have a Ferroli Domina F24E boiler that stops (block) after couple of seconds... I didn't open the combustion chamber because I don't know whether it is safe or not but from its cover's small window I can see a blue flame before the block. In the manual there is written to check the following: 1) gas input I have seen the flame so I guess it is OK