I used the Guiding Assistent from PHD2 which can also measure DEC backlash:
And PHD2 measured 1008ms backlash compensation!!!
After I entered this in the backlash compensation settings, I had a couple of "overshootings" where PHD2 correct in DEC so much that the star went to the other side. I asked my friend Tim Kahn about it who told me to only use 60% of the correction to avoid this. I did that, and since then have MUCH better DEC corrections.
I still wonder if I never noticed that before (would be strange) or if something really changed in the mount...
But the result is that I can now track REALLY well. I get 0.5 arcsec accuracy on pretty much all nights now.