Folder problem

one of our users found that all of his mail appear back to inbox today after filing into the folder few months ago. Mail are not unread nor mail rule is set in his mailbox. He has local replica in his laptop which scheduled replicate every minute daily. User’s access log is found that user has done read and write on the mail db at the time which incident is happened. This happen twice this week but not happened before since June 06 which this user is created. No major change is made by server or client.

Pls kindly help to troubleshoot

user environment:

notes client 6.5.5 with local replica but no archive

R6 mailbox design

6.5.4 domino server