Search on Exchange not working with EventID 1012: "Service at the endpoint address 'net.pipe://localhost/3867' is unavailable"
I recommend you follow the steps in this article Exchange 2013: FAST Search Technology Failed
Or what about creating a new DB and moving the mailbox to this new DB? Are there any errors?If you can move this mailbox, then test the moved mailbox in OWA.
Hope it works.