Had the same problem. After trying different smb.conf options, googling and reading the release notes of the changes, I tried upgrading to unstable package (2:4.4.3+dfsg-4 at the time of writing). This fixed my problems, I could revert all the changes I had made to the smb.conf.
I’m not sure if this is the same problem though, since in my situation I could access guest shares from Linux, Windows, but not OS X. I suspect my issues was more similar to: https://bugs.launchpad.net/ubuntu/+source/samba/+bug/1572301. Maybe upgrading to a newer version also helps to locate the problem.
