Overview:
Vault Admins can create, refresh, and delete sandboxes through the Admin UI. However, Vault admins encounter an error: This Vault's FTP Server is used as the source for XXX. Please ask your administrator to remove the link when refreshing a sandbox Vault.
Root Cause:
The File Staging (FTP) server of the Vault that is expected to be refreshed is linked to another Vault which blocks the refreshing.
Solution:
Re-link the File Staging (FTP) server to the specific Vault ID itself that in the error message under the Admin > Settings > File Staging Settings page in that Vault. More information about how to do this can be found on the Vault Help Documentation page under the Related Documentation section of this article.
Related Documentation:
Vault Help Documentation: File Staging Linking