Overview:
When trying to refresh a Sandbox Vault (Admin --> Deployment --> Active Sandbox Vaults --> Refresh a Vault), the Unexpected Error shows up.
Root Cause:
The error occurs because the Sandbox Vault is the source of a target Vault for FTP.
Solution:
Open a ticket with the Product Support team to unlink the FTP connection in the target Vault.
Related Documentation:
Article: Unexpected Error appears when trying to refresh a Sandbox Vault