ip6tables returned non-zero exit status 1 (unhandled exception on wrapper)

2014-05-08 17:08:44,023 - DEBUG    - L#2056 : leap.bitmask.gui.mainwindow:_cleanup_and_quit() - About to quit, doing cleanup...
2014-05-08 17:08:44,024 - DEBUG    - L#1506 : leap.bitmask.gui.mainwindow:_stop_imap_service() - Waiting for imap service to stop.
2014-05-08 17:08:44,027 - ERROR    - L#2067 : leap.bitmask.gui.mainwindow:_cleanup_and_quit() - No instance of soledad was found.
2014-05-08 17:08:44,028 - DEBUG    - L#2069 : leap.bitmask.gui.mainwindow:_cleanup_and_quit() - Terminating vpn
2014-05-08 17:08:44,029 - DEBUG    - L#2075 : leap.bitmask.gui.mainwindow:_cleanup_and_quit() - Cleaning pidfiles
2014-05-08 17:08:44,031 - DEBUG    - L#2152 : leap.bitmask.gui.mainwindow:quit() - Bye.
2014-05-08 17:08:44,033 - DEBUG    - L#53   : leap:write() - 2014-05-08 17:08:44-0500 [-] Stopping worker...
2014-05-08 17:08:44,036 - DEBUG    - L#33   : leap.bitmask.gui.twisted_main:stop() - Twisted reactor stopped
2014-05-08 17:08:44,036 - DEBUG    - L#36   : leap.bitmask.gui.twisted_main:stop() - Done stopping all the things.
2014-05-08 17:08:44,097 - DEBUG    - L#388  : leap.bitmask.services.eip.vpnprocess:_seek_to_eof() - Could not read from socket. Assuming it died.
2014-05-08 17:08:44,097 - DEBUG    - L#650  : leap.bitmask.services.eip.vpnprocess:_cleanup_tempfiles() - cleaning socket file temp folder
bitmask-root: ERROR: Could not run ['/sbin/ip6tables', '--delete-chain', 'bitmask']: None
Traceback (most recent call last):
  File "/usr/sbin/bitmask-root", line 396, in run
    subprocess.check_call(parts, stdout=devnull, stderr=devnull)
  File "/usr/lib/python2.7/subprocess.py", line 540, in check_call
    raise CalledProcessError(retcode, cmd)
CalledProcessError: Command '['/sbin/ip6tables', '--delete-chain', 'bitmask']' returned non-zero exit status 1
2014-05-08 17:08:44,417 - WARNING  - L#317  : leap.bitmask.services.eip.vpnprocess:terminate() - Could not tear firewall down

(from redmine: created on 2014-05-08, precedes #5588 (closed))