Shouldn't the stove reset its self when the temps come back down? If the high limit aquastat was tripped at 200, then after a hour or however much time passed water temp comes back down to 175, shouldn't the fan come back on and keep fire going? Was there still wood left in the firebox when the fire went out or had it all burned up? I had a fire almost go out on the coldest night of the year this winter and I believe it was from burning a little too wet of wood, and I hadn't taken out ashes in awhile. water temp had gotten down to 100 or 120.
Do you have more than one gauge to verify the temp gauge on the front of the boiler is correct? Maybe it is actually overheating and the gauge is wrong.
My stove is the same, if aquastat is set at 180 the temp gauge front of stove never shows 180, its usually 170 when the fan turns off.
The only time mine has tripped the 200 degree high limit safety is when the flapper door stuck open in warm weather, I think the 400 has two fans, make sure power is off to the fans, take them out and work the flapper open and closed by hand and see if there is any binding. Mine was being held open by cresote, it was tough to diagnose the problem because by the time i worked the fan out of the mounting hole it would free up the flapper from being bumped around.
Why were the two aquastats replaced in november? Similar issue?
Does the bladder have water in it, bladder valve is open, ashes are empty?