Maybe you already have, but I’d lean towards the selector board first. Besides, if you’re not 100% and you’re going roll the dice, the selector board is cheaper and easier to replace.
I’ve experienced the selector blipping twice in a step down from 3 to 2 causing a COR run. But after three recoveries, it’s down on on STLD (IIRC) hard fault. Your scenario is different, but….