I've checked the bellhousing to engine bolts and the gearbox to bellhousing bolts, but they were not loose.
The BW35 torque converter to flexplate bolts can be checked only with the engine out of the car. However I can't imagine the torque converter has come loose, because the problem does not occur after a cold start. When it has warmed up and I stop the car, it starts shaking as soon as it stands still (with the selector in D). But when I subsequently switch the selector into N, leave it there for at least one minute and switch back into D, the shaking is over (until the next stop).