Well, no, I hadn't. Or at least it wasn't checked when I took a look. I turned it on after reading this thread (after upgrading to version 12), then turned it back off again because I couldn't see a difference regarding the line spacing.
In the meantime I installed the 12.1.1 beta version and rebooted my machine (which I had also done after upgrading to Directory Opus 12), but it didn't help. I rebooted once more because of other reasons, and after reading your last posting, I turned "Use visual style to draw items" on again. Well, this time it did make a difference...
Strange thing. Also, I did indeed have to make almost all font sizes 1 point bigger (9 -> 10 pt) to get the original display back. But why?
Now everything looks as nice as before again, so I'm very glad and enjoy the great new features Directory Opus 12 brings (two of which I had really missed before).
Thank you very much for your help!