You will see similar errors below while running the Mailboxes Content and Attachment Gathering.
An unexpected error occurred during the gathering of information store content and attachments. 0x80004005: Unspecified error : .
The task processor which caused the error is the following: Component:Gathers content and attachment information from information stores (private and public) on an Exchange server.:
The following error occurred before the main processing of the given gathering: Unspecified error.
1. Turn of User Account Control (UAC) and reboot the MessageStats server.
2. Delete the existing Mailbox Content and attachment gathering and create a new one.
3. If you are gathering against a Exchange 2010 Database availability Groups (DAG) make sure that the source is the Active node of the DAG.