We lost the whole day because some DBA changed permissions on the Oracle directory and didn’t tell others. We simply could not login with sqlplus after setting (exporting) ORACLE_SID environment variable. The error we were getting were all pointing to various TNS related issue(s).
Some of us were able to connect, and we were able to work with it, if we used Oracle id. This sort of pointed to permission issue. But where? Turned out to be Below post clearly specifies the remedy:
Solution for ORA-12547 issue on Oracle database – TNS lost contact when try to connect to Oracle
I wish I saw the below post earlier. Hope it helps anyone else going through similar issue.