Fixed it!
I saw that my previous schedule item (a top up charge) ended at 1600 and this one started at 1600. I adjusted the export to start at 1601 and it's gone through fine.
I used to have it export from 1700 and that was always fine. I only changed to to 1600 (now 1601) because of the extra sun we're getting now.
Felmer - maybe you have a similar setup with identical times in the schedule?