Hello WiVM,
The items moved to this EVPM folder, are regular emails or Enterprise Vault shortcuts? Looking at the dtrace entries you posted, there is only one thread [8464] and it's pulling messages from the A6 queue
3591376 13:56:06.122 [8464] (ArchiveTask) <9028> EV:H {AgentMessageDispenser::ProcessNextMessage:#1013} Retrieved a [MsgID_ProcessMovedItemsInFolder4 (89)] MSMQ message on queue [EVPRD01\Private$\Enterprise Vault Exchange Mailbox Task for MBXPRD01 140213112233 A6] and it's retry count [0] is within retry limits.
The A6 queue is used for: "Requests to update folders with items that have been moved inside a mailbox" (TECH164832). if you are moving regular emails, then you are looking at the wrong queue, because this queue is used for shortcut moved from one folder to another.
Also, you need to confim in the dtrace if there is another thread trying to process the same message and/or mailbox. That would explain these entries in the dtrace. Can you post the entire dtrace and let us know what is the mailbox you used for the test?