A boiler or combi unit needs stable and safe electrical power. A weak socket, missing grounding, loose contact or voltage fluctuation can cause resets and damage to the control board.
An electrician should check the socket, grounding, breaker, RCD, panel contacts and voltage under load. A boiler should not be permanently powered through an extension cord or random socket.
If the boiler resets, the socket gets warm, the breaker trips or lights flicker when it starts, the electrical line should be professionally checked.