Yep, subject says it all: when a song in BIAB plays a 'C', I have to play 'D' on my MIDI keyboard.

I've never noticed this before because I normally record my piano playing in a DAW, and there everything's correct.
The BIAB MIDI-settings show my MIDI-keyboard as Input Driver, and the Microsoft GS Wavetable Synth as output driver.

Any ideas?