Database Mirroring: Check the network address name and that the ports for the local and remote endpoints are operational. (Microsoft SQL Server, Error: 1418)
The above error is quite common within the database mirroring setup, you could get to see.
The intial workaround for this error will be to check whether any network issues are causing this error when the failover is intiated from a Primary server to the Mirror server. By default for the Database Mirroring servers you have setup the endpoints to listen on the different ports, which is correct. Also test the connection with a simple TELNET connection test from the command prompt.
I have seen few installation where they are still with Service Pack 1 for SQL Server 2005 or RTM where you have to setup with Trace Flag 1400 in order to initiate the database mirroring session. Further it is always best to check the privileges for the SQL Server services account and ensure they have SYSADMIN privileges. From the error message text it refers "TCP://MYMACH.mynet.net:5022" can not be reached or does not exist", so you have to check by testing that instance and ensure to listen on the 5022 port, take a trip to SQL Configuration Manager for all the 3 instances involved in the database mirroring. In this regard I strongly recommend the users to refer through the "Troubleshooting Database Mirroring Setup" topic in BOL that is specially talks about versions of operating systems you are dealing with.
Say if you have had this error during the failove session then ensure to initialize the mirror by performing a full backup of the database on the princpal and restore the same full backup to the mirror with NO_RECOVERY. For the latest match of LSN on the transaction you must also restore any other log backups that are taken on the principal to the mirror.