TNS-12555: TNS:permission denied| TNS-12560: |TNS:protocol adapter error |TNS-00525: Insufficient privilege for operationLinux Error: 1: Operation not permitted
Whenever the oracle user permissions are modified or deleted from OS,the authentication of oracle user would be revoked
We can find some temporary debris socket files of the old oracle user listener permissions to access the oracle home

As a quick fix,delete all the temp files generated by listener in /var/tmp/.oracle directory which was used earlier
The key file placed on the directory which fails authentication
LISTENER =
(DESCRIPTION_LIST =
(DESCRIPTION =
(ADDRESS = (PROTOCOL = TCP)(HOST = orcl19x)(PORT = 1521))
(ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1521))
)
)


So these socket files should be deleted by root user because oracle already use these processes and dont have permissions to delete these files
Before deleting these files just take a backup copy of all the files in case of any issues

Cleanup the files

Listener is started successfully!

These socket files are used by listener process to exchange data to other oracle processes
