@Noel96: Thank you for your suggestions. I've never had a softsynth set up, as I never use it. When I have a song with midi-only exporting is no problem. The problem is exporting the Real tracks.
So I thought Audio Settings might have something to do with it (#6 in your screenshot). I have 3 options there: MME, ASIO and WAS. But no matter which one I set up, the problem is the same.
What bugs me most is that I hadn't changed anything in the setup; I'd just not used BIAB for a couple of months. So it seems logical something changed on my PC but what?