When trying to index an AD User Group from within the DLP Enforce Console with the "Include Mail Aliases" checkbox checked, the index will fail. This is happening whether it is reindexed on a schedule or manually when checking the "Refresh group directory index on Save" checkbox in the User Group.
Data file was corrupted (Creating Index Version xxx)
The issue is caused by a defect in the IncrementalMembershipMapper class.
DLP Version 15.1 and beyond.
Uncheck the "Include Mail Aliases" checkbox within ALL User Groups in the DLP Enforce Console, save the groups, and then reindex. The error should now be gone.
DLP 15.5 MP2