Are you getting "Validation Error. Please check the indicated fields below." when trying to update your support credentials in EBS 12.x OAM ? When you check the oam log (Site map -> Others -> Application Manager Log) do you see?:
javax.net.ssl.SSLKeyException: Hostname verification failed: HostnameVerifier=weblogic.security.utils.SSLWLSHostnameVerifier, hostname=updates.oracle.com
If so then the problem might be that the verifier on your oacores is wrong. Recently I added several oacores to our EBS 12.2 system and after doing so I was no longer able to update the password for my oracle support credentials. To fix you will need to login to the weblogic console and modify hostname verifier for the new oacores.
As you can see the oacore server1 is using a Custom Hostname Verifier. The newly added oacores are not though and will need to be modified to use the custom one. If not then whenever you connect to the oacore that isn't oacore1 you will get that error. The more oacores you have, the more likely you will be to get the error.
Hope that helps.