No more dropouts during plain audio playback, it seems!
Yamaha Steinberg FW Driver Utility (3.) was the key. After checking all those - and literally every other old times computer voodoo stuff - combinations it is/was changing IEEE1394 Buffer Size from Small [Default] to Large in the end.
The kudos goes to Felipe Garcia for providing the crucial link. Thanks a lot.
PS: don’t ask me why this change is required after the Win 10 upgrade has been done on the very same hardware.