In some scenarions the current Exchange cumulative updates (Exchange 2013 CU14, Exchange 2016 CU3) resulted in failed and corrupted search indices.
In Exchange Server 2013 the failed search service resulted in the following event log message:
Watson report about to be sent for process id: 28160, with parameters: E12IIS, c-RTL-AMD64, 15.00.1236.003, M.E.Search.Service, M.E.Data.Directory, M.E.D.D.ScopeSet.GetOrgWideDefaultScopeSet, System.ArgumentNullException, 301, 15.00.1236.000. ErrorReportingEnabled: False
A community post states that there won't be an interim hotfix.
But at least the issue has been found and will be fixed with the next cumulative updates for Exchange Server 2013 and Exchange Server 2016.