I tried simple name, i.e. TEST. It didn't work. I also tried TEST.HMTRETURN and TEST.HMTRETURN.COM. Didn't work either. Moreover, I tried to put just dummy name. i.e. FOO. I was still getting the same error: "Error while trying to retrieve text for error ORA-00000" !!! It refused to understand any Oracle Service Names from Tnsnames.ora, yet SQL*PLus uses it correctly.

Mark. could you tell me which version of the database, client and ODBC driver you have in your working combination ?