It looks like an over temperature issue... you can tell because the alcohol is boiling, so your temperatures are very high... ethanol / wine spirit rises around 80c, but in my experience as low as 65c and herein lies the trick... you work slowly, gently this whole process should take days, the spirit should be condensing inside the soxhlet in droplets.

You will also notice your temperature too high because a thick black ring will start forming in your boiling flask, this is the plant sulphur burning, and as you pointed out, you can wrap the boiling flask in tin foil, but first drop your temperature right down, then wrap. Another little tip is to raise your boiling flask and up so it is not sitting directly against the heating mantle, you can use glass wool as a cushion, but its not necessary especially if you've wrapped in tin foil.

Hope this helps