This issue has been resolved in the following release:

Enterprise Vault 9.0.3

Workaround:
The Case / Department Default Output folder is limited to two hundred and fifty six (256) characters.
1. Open the properties page of the Case and change the output folder variable.
2. Save the changes
3. Perform the export

Symantec Corporation has acknowledged that the above-mentioned issue is present in the current version(s) of the product(s) mentioned at the end of this article. Symantec Corporation is committed to product quality and satisfied customers.

There are no plans to address this issue by way of a patch or hotfix in the current or previous versions of the software at the present time. However, the issue is currently scheduled to be addressed in the next major revision of the product. Please be sure to refer back to this document periodically as any changes to the status of the defect will be reflected here.  Please note that Symantec Corporation reserves the right to remove any fix from the targeted release if it does not pass quality assurance tests.  Symantec’s plans are subject to change and any action taken by you based on the above information or your reliance upon the above information is made at your own risk.