Insufficient Storage Space: Titanium Backup

Weird problem,

I have 2 schedules a week, one that does full backup of new apps and old apps and the other just doing the old apps.
If I back up an individual app - OK
The "redo" of old apps - OK
the Full Backup - I get the error message: Insufficient Storage Space: Titanium Backup Error only pops up after awhile, so it can go 20% without problems, I've seen it get to 50% too..

Troubleshooting done:
-unchecked "mount namespace separation in SuperSU
-manually backed up all apps to see it wasn't an issue with the apps.
-I have also actually managed to manually trigger the full backup and it's been successful (but also get this error message sometimes)- But never been successful -from auto backup.
-It's not app specific because the error can come up for anything, even though it has been sucessful before.

It was working fine on Lollipop.
Since 6.0 it's been a problem
just upgraded to 6.0.1 and it's still the same.

I'm using a Sony Z5C

Any suggestions would be most appreciated!

Thanks

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