OK, my bad.

I downloaded a lot of the new RealTracks, but I didn't download the new ones for RealTracks 1-39 right away because it was taking a lot of time.

I was using _BOSMEDL, a multistyle from the 2009 RealTracks RT16. Once I downloaded RT16 from 2012 and overlaid the old one, the problem also went away. It's still sort of there (the measure sounds quite open, the bass is on 1 and 3, and the guitar just strokes the first beat on the beat). But at least it's not really in-your-face like it used to be.

And, thanks to Peter's timely suggestion, I've got a workaround that makes it sound a lot better (Dm7/C instead of Dm/C).

So all's well that ends well.

Thanks.