User was authenticated but not found in database
The customer was often seeing this kind of error in the logs :
2016-06-27 12:39:22,815: ERROR [JcrSessionFilter] - Error while authorizing user org.jahia.pipelines.PipelineException: user 'XYZ' was authenticated but not found in database!
after clarification, the error came when a user is back after a while.
The result of the JCR integrity tool has revealed a huge number of messages of this type: "Couldn't find referenced node with UUID ..." in the live and default workspaces.
A try with the "Fix full Java Content Repository integrity" option of JCR integrity tool, combined with a re-indexation of the JCR repository has solved the issue.