There are a number of support articles on this subject (KB53607,KB53461,KB69850,KB51640 for example).
But they generally all boil down to the same thing - something is preventing connection to the Sql database to validate your ePO credentials.
It might be the credentials are wrong, or maybe the Sql server has has some changes to its accounts, or maybe the sql ports have changed or a number of other scenarios with the same result.
If https://localhost:8443/core/config-auth doesn't work when executed on the ePO server directly it does suggest something has happened with the db.properties file on the ePO server that drives this url.
Do not post db.properties to a public forum such as this though, it contains network-specific information you would not want to share with the world.
If you want that analysed, open a McAfee support case instead.
Bear in mind there is a default 'admin' account for ePO, you just need that password.