I recently updated to 2.06 (2.06+ Build 150723) and now ranking question doesn't work anymore.
I thought the issue would be that I had a min and max number, but even without any min/max, I get the popup, that a question is not answered valid. (The question is not set mandatory)
Even if I choose every option, I can't submit. Only if I don't drag any answer to the right side, i can proceed.