Actions that call iptables fail when they are run at the same time by separate threads. I have added a lock to disallow prevent the problem. The patch should be applied to server/action.py
There is another patch for this problem at https://sourceforge.net/tracker/?func=detail&aid=2857096&group_id=121032&atid=689046 but it is more extensive and causes another problem detailed in the link provided. My patch changes as little as possible to fix the problem.