Thank you for your prompt reply. In the meantime, I had a chance to try the calibration for a colleague instead of myself and that did seem to work. So it might be about the set-up (I mean lights, distance, etc.) after all. After multiple attempts (with different distance, lighting, etc.), it worked for me as well. I was just a bit surprised because we calibrated for over 50 participants before the summer and never experienced it being so sensitive to these things. But for now, it seems like our problem is solved if we just plan a little bit more time to make sure everything is set up correctly.
Thank you again,