If you see the following error in the alert log of the new primary server during a switchover, that’s normal. It’s a transient one.

********************************************************************
 Fatal NI connect error 12514, connecting to:
  (DESCRIPTION=(ADDRESS_LIST=(ADDRESS=(PROTOCOL=TCP)(Host=OL6DB01)(Port=1521)))(CONNECT_DATA=(SERVICE_NAME=PROD1)(CID=(PROGRAM=oracle)(HOST=ol7db02)(USER=oracle))))
 VERSION INFORMATION:
         TNS for Linux: Version 12.1.0.2.0 - Production
         TCP/IP NT Protocol Adapter for Linux: Version 12.1.0.2.0 - Production
   Time: 30-OCT-2020 15:08:40
   Tracing not turned on.
   Tns error struct:
     ns main err code: 12564
 TNS-12564: TNS:connection refused
     ns secondary err code: 0
     nt main err code: 0
     nt secondary err code: 0
     nt OS err code: 0

What happens is during the switchover, the old primary will be shutdown and brought up as the new standby. The services registered previously in the listener will be removed and registered daynamically again when Oracle starts up on the new standby. During this period, you will see the errors and they will be gone once services are registered in the listener.

• Oracle