Jamie, I've had the same issue, and this - ie. removing the ipv6=1 line - has fixed it. The issue came up right after installing webmin from the repository, it didn't even start the service (of course) just put the error messages into syslog. I was testing it on a Amazon Web Services EC2 micro instance with Ubuntu 16.04 where ipv6 was disabled. All is good with this fix now.
VOTE: lazarus