Ok, it seems I have resolved the issue by allowing a response interval of 1 second instead of 0.25 seconds.
I installed FDC on a second computer via WideFS, and with my profile for FDC it displayed the same error - so I was able to rule out the soundcard or the driver. As I noticed it ran fine with the default response time, I applied this time to my setup and - voila.
Thanks for assisting me.