I had this problem when I copied and pasted a snapshot of another account to a different account.
Not sure if the snapshot was originally corrupted or not, but by using "current condo snapshot" I was able to restore it.
If you didn't do this, it's most likely another snapshot bug.
Even without swapping them between accounts, TU snapshots are very unreliable (I've had them being corrupted multiple times in the past) even on low item counts.
Never trust them at all and save into multiple snapshots and make separate backups.