I had the same problem with some Kodi addons being the culprits. Updated python-openssl and python-cryptography to the packages from the debian repo, disabled some theme/addon helper that was running in the addons settings and the unity7.log stopped filling my disk. Let me see if I find the link to the bug on Launchpad.

Edit:

https://answers.launchpad.net/ubuntu/+question/447828

https://bugs.launchpad.net/ubuntu/+source/xorg/+bug/1636573