wrong type argument

[Mahdi Osman]

Sorry again for my question.
I am using emacs 21.3 and I have the following problem.
Sending mail using gnus gives me "wrong type argument: stringp, nil" and
the cursor jumps to "From:" which is me. My mail is rejected and sent to
 draft.
All my incoming mails are marked "O". Even after unmarking them, when I
restart my emacs and gnus, mails are still marked "O".
...............
The following the trace I found:
;;+OK AVG POP3 Proxy Server <23908.1114410983@mp026> 7.0.307/7.0.308 
[266.10.2]""
;;+OK May I have your password, please?
;;+OK mailbox has 0 messages (0 octets)
;;+OK mailbox has 0 messages (0 octets)
;;.
;;+OK bye
Process POP exited abnormally with code 256
..........
Finally I can not exit gnus and it emacs hangs up showing "wrong type
argument: stringp, nil" in the mini buffer.
I hope somebody can give me a hint on how to overcome these problems.
Thanks a lot
-- 
Mahdi Osman (PhD)