i had a identical call out last week on a vokera linea 28, scratching my head for a while checked thermisters they were ok, and it only seemed to happen on hot water or when heating got up to about 60 degrees...
any way changed the ignition pcb on side of gas valve and that fixed that..
pipe to pressure vessel may be blocked - unblock
or
possible leak on schreider valve on pressure vessel
or
calorifier broken down allowing dhw to enter primary water circuit. - replace calorifier.