When journal archiving the message that EV determines to have an invalid "Original Message Class" then Storage Archive process will restart during attempt to archive the message.
Category: Storage Server
Description: Storage Archive Server (1) - Restarting failed process
Exit Code: <0xc0000417>
The process has failed abnormally and will be restarted automatically
12551 10:09:05.393  (StorageArchive) <6564> EV:M Original Message Class found: (null)
This issue has been addressed as part of the following release:
Enterprise Vault 9.0.1 - Release Details