EV Discovery Accelerator (DA) has the capability to place Legal Holds on Savesets in the EV Vault Store databases for messages found and accepted in a DA search. This process normally works in the background until all accepted search hits for the appropriate DA Case are on Legal Hold.
When the EV Vault Store databases need to be backed up, they need to be placed into Backup Mode. Doing so while the DA processing is attempting to place Savesets on Legal Hold, can result in the Event ID 6621 being continuously thrown into the EV Event log of the EV Server as long as the server is in backup mode.
This is an example of the Event ID 6621:
Source: Enterprise Vault
Category: Storage Online
Event ID: 6621
An exception has occurred.
Reason: The Vault Store is currently in Backup Mode. [0xc0041b85]
No errors are logged on the DA Server.