1. In PokerTracker4 click "Configure -> Options" and if "Download Premium Content" has a check mark, click it such that the check mark goes away.
2. Click "Database -> Database Management -> Housekeeping -> Revert PostgreSQL Tuning".
When those are done close things down and reboot.
3. Retry "File -> Backup" once more but clear all check marks under "Optional" (those can be backed up afterward or manually if required).
Also when creating the backup zip file name make sure to save to a disc on the same machine as your PokerTracker 4 e.g. directly to C:\. For example trying to send it to a network share like a Dropbox folder could create trouble. Once the backup is created you're free to move it wherever you like.
4. Presuming the backup now succeeds you can reverse the check mark from step #1 to turn your premiums back on if applicable and repeat step #2 but this time select "Tune PostgreSQL".
If you still get an error when backing up please open a
Support Ticket and
report your problem so that we investigate that further for you.