Solution to ORA-28547: connection to server failed, probable Oracle Net admin error

When trying to perform sqlplus actions against one of my databases, I received

oracle@oric-dbserver01:[mydb01]# sqlplus / as sysdba

SQL*Plus: Release 19.0.0.0.0 - Production on Wed Apr 17 10:43:56 2024
Version 19.22.0.0.0

Copyright (c) 1982, 2023, Oracle.  All rights reserved.

ERROR:
ORA-28547: connection to server failed, probable Oracle Net admin error

Cause:

This is a consequence of the fact that oracle nls files are not found in its default location for this particular server.

If there are no .nlb files in $ORACLE_HOME/nls/data, you need to find out where they are located and set the ORA_NLS10 parameter correctly

If you lookup the error you will get a hint about this fact:

oracle@oric-dbserver01:[mydb01]# oerr ora 28547
A failure occurred during initialization of a network connection from a client process to the Oracle server.  ... 

The most frequent specific causes are: [ oracle lists several potential causes here ]

The character set of the database is not recognized by the client process, which may be caused by an incorrect or unnecessary setting 
of the ORA_NLS10 client environment variable or by a new or user-defined character set installed in the Oracle server and used for the database.

Solution:

Set the ORA_NLS10 environment variable:

export ORA_NLS10=$ORACLE_HOME/nls/data/9idata

When this is done, sqlplus will work.

Don't forget to add the same to your .bash_profile for the operating system user owning and running the oracle server software. For most installations I have seen, this will be the os user called "oracle".