Major Description
There is a sync issue in TransactionBroker, that can result in contention on the xatransaction map.
The result is a get returning null for something present. So an Ack cannot resolve the corresponding transaction.
The error - transaction not started - xaErrorCode:-4
Either a resize or unrelated remove op could cause inconsistency and presumably allow a transient null.
Trying to reproduce reliably in a test case is elusive.