Titanium Backup "insufficient free storage space" error
This is due to the 'Mount namespace separation' setting in SuperSU being turned on (by default). Untick that, reboot and try your Titanium Backup operation again.

This happens because the permissions to app-created folders are restricted in newer Android versions, I believe. Ironically, I didn't have this issue on Marshmallow-based CM13, but I did face it in the Lollipop-based OxygenOS for the OnePlus One.