Error 6 initializing SQL*Plus Message file sp1<lang>.msb not found SP2-0750: You may need to set ORA

This cgi application server has worked 3 years without problem to connect database version 12.1.0.2. in window environment. oracle client both 32 & 64bit were installed on it. We recently upgraded one part of 3rd party software, everything works fine except the error above, error 6 initializing Sqlplus message, with SP2-0667 and 0750. We checked the environment variable setting (path & oracle_home) seems correct. tnsnames.ora file no change. Don't know what else I should do. I checked registry key, no additional "\"

I am looking for your idea I can try it out before I reinstall oracle client software. New installation may not solve the problem. Thank you for any help in advance. -- Vivian