while connecting with "system" occures ORA-01821 error

All,
I set up the command "sqlplus system@xxxx" to open a sqlplus session from a Oracle client to its server. The error message I get is "ORA-01821: date format not recognized" and on a second line the error "SP2-0152: ORACLE may not be funtioning properly".
What has an date error to do with that kind of session? I already verified the tnsnames.ora and sqlnet.ora files, everything ok. I also can tnsping the server, everything fine.
Thank you for every comment.
Regards,
Thomas

What are the OS environment variables NLS_LANG and NLS_DATE_FORMAT set to?
Beksy

Beksy,
the NLS_LANG variable is set to AMERICA_AMERICA.WE8ISO8859P1. The NLS_DATE_FORMAT variable doesn't exist on this system. But on another client machine that works fine, this parameter doesn't exists too. Is there an other place where I need to set the date format in an Oracle client?
Thank you for you answer.
Regards,
Thomas

Thomas,
Just a guess!
Shouldn't that be AMERICAN_AMERICA.WE8ISO8859P1 ?
(notice the 'n')
Is it the same in both server and client?
Beksy

Beksy,
sorry for that typing error(in that message I sent to you, not in the registry!). You are right it has an N. It's the same on both machines, yes. Thank you for your help. Any further ideas?
What you think, uninstall the client, clean up the registry and reinstall the client could solve that? I am in a hurry with that because it's on a client machine. So I need to solve that ASAP, do you understand?
Regards,
Thomas

Thomas,
Have you tried to use the GUI version of sqlplus?(if you are using windowsNT)
Try to set the NLS_LANG parameter just before starting the sqlplus from dos.
What operating systems are you using for client and server?
Beksy