Hello Guenter,
Thanks for the feedback. My suspicion at this stage is the hardware: some problem with the PCI bus on an old-ish machine I've built for an installation. As I recall, even the Sound Blaster card wouldn't work when the Hammerfall was present, but the Sound Blaster works fine when the Hammerfall is removed. For now we'll just go with Sound Blaster analogue stereo, since we're under a tight deadline, and will look again at the Hammerfall when there's less time pressure.