
That's not a "corruption indicator"; it is an indication that the other side of the Transfer no longer exists. Quicken handles this smoothly; it's like a memo of what had happened, but the transaction has no category and doesn't transfer money to another account. It certainly could have left a transaction in this state when the computer crashed during entry. (I created this by adding a dummy account, creating a transfer to that account, and then deleting the dummy account; thee other side of the transfer transaction remains, with a broken transfer indication.) If you got some other icon, I'd be interested to know what it was.
I have no explanation for you not being able to delete this transaction, and then suddenly you could. Modern SQL databases are generally able to prevent damage and self-repair, but this sounds a bit odd.