Recently I encountered an issue with the mirroring where all databases are in disconnected state. I checked the space on the drive and all seems fine, even I initialized the endpoints by starting and stopping and then tried to resume the mirroring, but no luck.
I checked the SQL error log and found below error which says permission denied on end points.
Database Mirroring login attempt by user ‘SQLServiceAccount.’ failed with error: ‘Connection handshake failed. The login ‘SQLServiceAccount’ does not have CONNECT permission on the endpoint. State 84.’
This message actually informs that your security configuration for the mirroring protocol is broken. Hence you can easily rectify this error by fixing the permission on endpoint using below syntax:
Grant Connect on Endpoint::[endpoint name] to [account]
Grant Connect on Endpoint::Mirroring to [GAR\sys_SQLAccount]