You didn't connect the power amp out to the monitor out accidently, did you? That would explain the error, I did that once to my 11 rack back in the days... buffer was gone - luckily it was an easy replaceable SO-08 housing.
Besides: as it is not the power section - the failure must have been in the power amp - there's no way a DA converter (or buffer) can burn a transistor.
It is weird though that both your kemper went down, as unlikely it is that both Kempers had a failure at the same time it is with the power amps...
That's one of the first things we checked. We turned everything off when the smoke started coming out of the Kemper. We traced all the lines. It was all properly hooked up.