Guys,

The error is misleading while the solution is pretty simple and straight..

Server A --> principal --> domain\Servic_Account1

Server B --> Mirror --> domain\Servic_Account2

Make sure domain\Servic_Account1 has sysadmin rights on Server B

and the same way domain\Servic_Account2 has sysadmin rights on Server A.

Set up the mirroring again and you should be good with out any errors.