The user gets an error stating that there are documents that reference the affected Milestone in Vault. When attempting to determine the affected documents, the user is unable to find them using Vault Reporting.
The documents that reference the affected Milestone have been archived.
To resolve this issue, the user will need to unarchive the impacted documents so that the milestones are removed and then delete the milestones and manually archive the documents.
Alternatively, if the business process allows it, the user could deactivate the milestone instead.
Vault Milestone Documentation: Archiving Documents