Timetablers please take note of this recent issue - it's that time of year when people start to sort out options and timetabling for next year, so it's worth checking out the issue and keeping an eye on SupportNet for updates:

Timetable Nova T6 "Blank" backup issue - KB404346

There is currently an issue under invetigation in Nova-T6 which has affected a few users.
Take care when deleting subjects used in Alternative Curriculum as the SPL can be blank and also the automatic backups.

After saving and re-opening an SPL in Nova-T6, the file is blank.
Timetable file has lost data
Automatic $ and % backups are empty

This problem is under investigation. This solution will be updated as soon as possible with details of a permanent fix.

In Plan | Subjects, please either:
avoid deleting subjects altogether
Go to Data | Alternative Curriculum and check for AC activities linked to subjects which are to be deleted. Delete all these activities and save the file before opening Plan | Subjects to delete the subjects. Please also make a copy of the SPL file before and after this process. Close and re-open Nova-T6 after deleting the subjects.

Additional Comments
The problem is associated with the deletion of Subjects which have been used in unscheduled Alternative Curriculum activities.

If a file appears to be empty, and a great deal of work has been lost, please send a copy of the latest apparently blank SPL and % or $ backup to your local support team. It may be possible for us to recover the data.