I've tried the same thing for support structures and found 0.25mm to be too small a gap. Try upping to 0.5 vertical gap and a 1.0mm horizontal gap. You can get away with a larger horizontal gap as your printer should be able to bridge this easily. Also, if your support sits on another part of your print, you'll need to make it (the support) hollow with walls of a specific width, or it won't come away from the bottom surface at all easily.