That happened to me too the first time I made it. I think it is from using too much Pau d'arco in the water. It is very pleasant to drink when you use less.
I pasted Janaki's post below with a recipe.
Thanks to everyone for the input!!!
Excuse me for budding in, I believe that Julia Chan at sensiblehealth.com sells it (I got some through her and have been told it's very good quality). There are some specific instructions for storing it (glass jar with a lining of paper under the lid). It should be prepared in stainless steel or glass pot (not Corningware) using stainless steel spoon. I believe it is one flat tablespoon for 3 cups of water. Boil 5 minutes and simmer for 20 minutes. I got these instructions from her as well. Julia is in Canada, so Andreas may know someone closer to you.
I just made some last night so it's fresh in my memory.