Database Mirroring login attempt by user 'Domain\Hostname$.' failed with error: 'Connection handshake failed. The login 'Domain\Hostname$' does not have CONNECT permission on the endpoint. State 84.'. [SERVER: XX.XXX.XXX.XX]

Solution:

1. Check whether the SQL server instance is running on a Service account or not.
2. if it is not running make sure to set the service account for all SQL server instances which is a part of Always on group.
3. GRANT CONNECT ON ENDPOINT::hadr_endpoint TO [Domain\user] for all SQL server instances.
4. Make sure to give Sysadmin Privileges to SQL server Service account for all SQL server instances which is a part of Always on group.