Torstan, tt is not the snap to grid toggle, it is the grid itself. When you change the grid spacing (at least in my version 2.6.4) you have to restart gimp, otherwise the snap to grid toggle continues to use the old spacing. I find it a pain to do this, as I often want to change the snap spacing while working (i.e. to get a 1/2 grid snap).

-Rob A>