I just checked if I had installed the Dibs Fix properly, and it turned out I had forgotten to change "YOURDOMAIN" to, well, my domain. Can't believe I was that stupid! :D Anyway, I'll see if that solved the problem.
Another theory: What if the problem comes from that some customers simply don't click "Next" after the payment is made. I would be reluctant to do so myself. One thinks that it's just one of those "Close" buttons, and then you just close the window instead on clicking the button to do so. To avoid this, I just added this text in the check-out procedure of my site: