ＴＲＹ： Please ensure that the SID name is actually the same (no typos)in the TNSNAMES.ORA file. Also, if your server is A Unix machine, be sure that the SID is in the same case as in the LISTENER.ORA file. Lastly, if you are connecting using a version 7 listener, be sure that you are not using the SERVICE_NAME parameter in the TNSNAMES.ORA file, you should be using SID=<sidname>.
Please ensure that the SID name is actually the same (no typos)in the TNSNAMES.ORA file. Also, if your server is A Unix machine, be sure that the SID is in the same case as in the LISTENER.ORA file. Lastly, if you are connecting using a version 7 listener, be sure that you are not using the SERVICE_NAME parameter in the TNSNAMES.ORA file, you should be using SID=<sidname>.