Well, this is kind of a complicated bug to describe, but the thing is that I cannot drag and drop windows, layers, echannels, paths, etc.
After installing GIMP this still worked, but since today it just stopped working
I had the same problem with my old computer, and there I didn't get it from the first day either
I haven't installed ANYTHING since the last time it worked
Reinstalling GIMP doesn't fix anything
Resetting the preferences doesn't work
deleting the '.gimp-2.8' folder in my user folder doesn't fix it
changing the window ode from 'Utility Window' to Keep Above DOES fix the problem, but after changing it back it stops working again.
on my old PC this fixed itself after some time, but for no reason.
Restarting windows doesn't fix it, but some times it just started working after I started the computer, so it's not some program that I started or closed that might be causing this.
A friend is also getting the exact same problem.

The only way to fix this seems to be reinstalling windows, and that is obviously not an option.

I didn't find any help on the internet and I think this bug hasn't been reported yet, but maybe I just didn't see it.