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Oracle Client + ORA-12557: TNS:protocol adapter not loadable

Deat experts, We migrated from Oracle 12c to Oracle 19c in the windows server 2016 last week.
The DBA did not remove the previous version of oracle client - 12c and in addition installed the latest version of Oracle 19c.
Also I can see the environment variable has below 2 entry in addition to both the client installation



When we connect to the database using external connection, we get the below error. The DB connection was working fine prior to the oracle 19c installation.

The program has ended abnormally due to an unhandled exception.
Exception Code (OCI[DBConnectOCI.cpp[192]]
ORA-12557:TNS:protocol adapter not loadable)

I am not a DBA person but technically oracle has what it needs, why does it not getting connected to the DB using oracle client 19c?
Is it 100% true that we need to remove the previous version of the DB Client and always have only one client version?
Is the environment variable creating the issue ? or both env variable and oracle client ?

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