The error messages below were captured from EV 11.0.0. Error messages may differ depending on the installed version.
No preview of this item is available (Error Code: 0x8007007F) If the problem persists, contact your Enterprise Vault Administrator.

To validate the issue a DTRACE is needed. Perform the following steps on the EV server while re-creating the issue.
For more information about Symantec Enterprise Vault’s DTRACE Utility please see Article 100038975.
DTRACE the process w3wp for no less than 5 minutes while re-creating the issue.
Open the log file created and search for either of the following the strings:
There are two conditions:
Note: After an upgrade of the Enterprise Vault server, the EV Reporting components must also be upgraded using the Enterprise Vault Reports Configuration utility (see the Enterprise Vault Upgrade Instructions document for more information about this utility). This tool updates the copy of EVRT.dll that is kept with the SSRS installation. The preview and export of items fails with error code 0x8007007F when the w3wp process loads an old version of EVRT.dll into memory instead of the updated version.
Perform the upgrade of Enterprise Vault Reporting to update the EVRT.dll
Applies To
This issue occurs in environments where SQL Server Reporting Services (SSRS) is installed on the EV server. SSRS is required in order to use EV reporting features.