[Office 2016] Search cannot complete the indexing of your Outlook data

Have various Outlook 2016 search problem recently, but couldn't solve the latest one. User is on Windows 7

What happen:

Everytime the user starts the Outlook, it will give this warning.

Search cannot complete the indexing of your Outlook data. Indexing cannot continue for C:\Users\xxxxx\Documents\Outlook Files\xxxxxx.pst (error=0x81404005). If this error continues, contact Microsoft Support.

Search will works, but will not index any new incoming emails

If user close the Outlook and start again, it gives the same warning again, and previous unindex emails will be indexed. But subsequently new incoming emails will not be indexed again until the next Outlook restart.

We have tried:

1. rebuild the index
2. create a new mail profile
3. move the pst to the totally new pst
4. update Office to latest version 1705, Build 8201.2102
5. run a repair Office

still having the same problem.

any suggestion?