Thanks for the cooling hint. Hadn't thought of that, but that's something to be aware of since my machine is overclocked ...
Anyway, I'm now pretty much sure that particular issure was a schedule task interfering. So I killed them all ;-)
Yet, I need to get some sort of a timer installed that recommends to end such lengthy session earlier, for the sake of beer. 9 hrs would have been enough because fresh ears revealed that the overdubs from the last 2 hours weren't that great ...